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This article is about contraceptive uterine devices that do not contain hormones. For hormonal uterine devices, see IntraUterine System.
An intrauterine device (intra meaning within, and uterine meaning of the uterus) is a birth control device also known as an IUD or a coil (this colloquialism is based on the coil-shaped design of early IUDs). It is a device placed in the uterus and is the world's most widely used method of reversible birth control,[1] currently used by nearly 160 million women (just over two-thirds of whom are in China where it is the most widely used birth control method, surpassing sterilization).[2] The device has to be fitted inside or removed from the uterus by a doctor or qualified medical practitioner. It remains in place the entire time pregnancy is not desired. Depending on the type, a single IUD is approved for 5 to 10 years use (the copper T 380A is effective for at least 12 years).[3]
[edit] Types of IUDs
There are two broad catagories of intrauterine contraceptive devices: metal-based devices, and devices which release hormones similar to those found in oral contraceptives. Although U.S. organizations refer to all types of uterine devices as IUDs,[4][5] in the UK the term IUD only refers to inert or copper-containing devices, and the hormonal uterine devices are considered a separate method of contraception termed IntraUterine System or IUS.[6][7]
Most non-hormonal IUDs have a plastic T-shaped frame that is wrapped with copper and/or has copper bands. Some IUDs, such as the Nova T 380, also contain a small amount of silver.[6] The arms of the frame hold the IUD in place near the top of the uterus. The GyneFix does not have a T-shape, but rather is a loop that holds several copper tubes. The GyneFix is held in place by a suture to the fundus of the uterus. All copper-containing IUDs have a number as part of their name. This is the surface area of copper (in square millimeters) the IUD provides.
Availability of IUDs varies widely by country. Only one brand of copper-containing IUD (ParaGard T 380A) is available in the
[edit] Effectiveness and mechanism of contraception
All second-generation copper-T IUDs have failure rates of less than 1% per year, and cumulative 10-year failure rates of 2-6%.[8] A large WHO trial reported a cumulative 12-year failure rate of 2.2% for the T 380A (ParaGard) (an average failure rate of 0.18% per year over 12 years), equivalent to a cumulative 10-year failure rate of 1.8% following tubal ligation.[3] The frameless GyneFix also has a failure rate of less than 1% per year.[9] Worldwide, older IUD models with lower effectiveness rates are no longer produced.[10]
The presence of a device in the uterus prompts the release of leukocytes and prostaglandins by the endometrium. These substances are hostile to both sperm and eggs; the presence of copper increases this spermicidal effect.[11][12] The same effect is believed to harm developing embryos. While the primary mechanism of the IUD is spermicidal/ovicidal, post-fertilization mechanisms are believed to contribute significantly to their effectiveness.[13] Because many pro-life groups define fertilization as the beginning of pregnancy, this secondary effect has led some to label the IUD an abortifacient.
Some barrier contraceptives protect against STDs. Hormonal contraceptives reduce the risk of developing pelvic inflammatory disease (PID), a serious complication of certain STDs. IUDs, by contrast, do not protect against STDs or PID.[14]
[edit] Contraindications
The WHO Medical Eligibility Criteria for Contraceptive Use and RCOG Faculty of Family Planning & Reproductive Health Care (FFPRHC) UK Medical Eligibility Criteria for Contraceptive Use list the following as conditions where insertion of a copper IUD is not usually recommended (category 3) or conditions where a copper IUD should not be inserted (category 4):[15][16]
Category 3. Conditions where the theoretical or proven risks usually outweigh the advantages of inserting a copper IUD:
· Postpartum between 48 hours and 4 weeks (increased IUD expulsion rate with delayed postpartum insertion)
· Benign gestational trophoblastic disease
· Very high individual likelihood of exposure to gonorrhea or chlamydial STIs
· AIDS (unless clinically well on anti-retroviral therapy)
Category 4. Conditions which represent an unacceptable health risk if a copper IUD is inserted:
· Immediately post-septic abortion
· Before evaluation of unexplained vaginal bleeding suspected of being a serious condition
· Malignant gestational trophoblastic disease
· Cervical cancer (awaiting treatment)
· Distortions of the uterine cavity by uterine fibroids or anatomical abnormalities
· Current PID
· Current purulent cervicitis, chlamydial infection, or gonorrheal STIs
· Known pelvic tuberculosis
Adverse reactions have been reported in women with metal allergies, both copper[17] and nickel.[18] An anecdotal report on a LiveJournal community group journal states that the wires wrapped around the T-frame of the ParaGard IUD are nickel wires plated with copper.[19]
While nulliparous women (women who have never given birth) are somewhat more likely to have side effects, this is not a contraindication for IUD use.
Some doctors prefer to insert the IUD during menstruation to verify that the woman is not pregnant at the time of insertion. However, IUDs may safely be inserted at any time during the menstrual cycle as long as it is reasonably certain the woman is not pregnant.[20] Insertion may be more comfortable if done midcycle, when the cervix is naturally dilated.[21]
[edit] Side effects and complications
Insertion of the IUD may introduce bacteria into the uterus. The insertion process carries a small, transient increased risk of pelvic inflammatory disease in the first 20 days following insertion.[14] It is very important that the provider use proper infection-prevention techniques during insertion.[22] Antibiotics should be given before insertion to women at high risk for endocarditis (inflammation of the membrane lining the heart), but should not be used routinely.[23]
During the placement appointment, the cervix is dilated in order to sound (measure) the uterus and insert the IUD. Cervix dilation is uncomfortable and, for some women, painful. Doctors often advise women to take painkillers before the procedure to reduce discomfort, and some will use a local anaesthetic.
After IUD insertion, menstrual periods are often heavier, more painful, or both - especially for the first few months after they are inserted. On average, menstrual blood loss increases by 20–50% after insertion of a copper-T IUD; increased menstrual discomfort is the most common medical reason for IUD removal.[24]
Complications include expulsion and uterine perforation. Uterine perforation is generally caused by an inexperienced provider and is very rare. Expulsion is more common in younger women, women who have not had children, and when an IUD is inserted immediately after childbirth or abortion. Women should check the string of the IUD at least once per menstrual cycle to verify that it is still in place.
The string(s) may be felt by some men during intercourse. If this is problematic, the provider may cut the strings even with the cervix, so they cannot be felt. Shortening the strings does prevent the woman from checking for expulsion, however.
The risk of ectopic pregnancy to a woman using an IUD is lower than the risk of ectopic pregnancy to a woman using no form of birth control. However, of pregnancies that do occur during IUD use, a higher than expected percentage (3–4%) are ectopic.[25]
Although the pregnancy rate during IUD use is very low (less than 1% per year), it is not a 100% effective method of birth control. If pregnancy does occur, presence of the IUD increases the risk of miscarriage, particularly during the second trimester. It also increases the risk of premature delivery. These increased risks end if the IUD is removed after pregnancy is discovered. Although the Dalkon Shield IUD was associated with septic abortions (infections associated with miscarriage), other brands of IUD are not. IUDs are also not associated with birth defects or other pregnancy complications.[26]
[edit] Use as emergency contraception
Intrauterine devices can be used as emergency contraception to prevent pregnancy up to 5 days after unprotected sexual intercourse, or sexual intercourse during which the primary contraception is believed to have failed (e.g. a condom was used, but it broke). Insertion of a copper-T IUD as emergency contraception is more than 99% effective, making it more effective than emergency contraceptive pills (ECP or 'morning-after pill').
[edit] Popularity
The ParaGard T 380A was approved by the U.S. Food and Drug Administration (FDA) in 1984 and became available for use in 1988. It is still the only IUD approved for use in the
Usage in other countries has been determined by surveys of married women of reproductive age. In this population, IUD use ranges from 5% in
[edit] Hormonal uterine devices
Main article: IntraUterine System
Photo of LNG IUS
Hormonal uterine devices do not increase bleeding as inert and copper-containing IUDs do. Rather, they reduce menstrual bleeding or prevent menstruation altogether, and can be used as a treatment for menorrhagia (heavy periods).
Although modern IntraUterine Systems use low doses of hormones, they still have the potential side effects associated with other hormonal contraceptives.
Progestasert was the first hormonal uterine device, developed in 1976[29] and manufactured until 2001.[30] It released progesterone, was replaced annually, and had a failure rate of 2% per year.[31]
As of 2006, the LNG-20 IUS - marketed as Mirena by Schering Health - is the only IntraUterine System available. First introduced in 1990, it releases levonorgestrel (a progestagen) and may be used for five years.
A lower-dose T-shaped IntraUterine System named Femilis is being developed by Contrel, a Belgian company. Contrel also manufactures the FibroPlant-LNG, a frameless IUS. FibroPlant is anchored to the fundus of the uterus as the GyneFix IUD is. Although a number of trials have shown positive results, FibroPlant is not yet commercially available.[32]
[edit] History
Presenters at a family planning conference told a tale of Arab traders inserting small stones into the uteruses of their camels to prevent pregnancy. Although the story has been repeated as truth, it has no basis in history and was meant only for entertainment purposes.[33]
Precursors to IUDs were first marketed in 1902. Developed from stem pessaries (where the stem held the pessary in place over the cervix), the 'stem' on these devices actually extended into the uterus itself. Also known as interuterine devices (because they occupied both the vagina and the uterus), they had high rates of infection and were condemned by the medical community.[34]
The first intrauterine device (contained entirely in the uterus) was described in a German publication in 1909, although the author appears to have never marketed his product.[35]
In 1929, Dr. Ernst Gräfenberg of
In 1934, Japanese physician Tenrei Ota developed a variation of the Gräfenberg ring that contained a supportive structure in the center. The addition of this central disc lowered the IUD's expulsion rate. These devices still had high rates of infection, and their use and development was further stifled by World War II politics: contraception was forbidden in both Nazi Germany and Axis-allied
The first plastic IUD, the Marguiles Coil or Marguiles Spiral, was introduced in 1958. This device was somewhat large, causing discomfort to a large proportion of women users, and had a hard plastic tail, causing discomfort to their male partners. The Lippes Loop, a slightly smaller device with a monofilament tail, was introduced in 1962 and gained in popularity over the Marguiles device.[34]
The stainless steel single-ring IUD was developed in the 1970s[36] and widely used in
Dr Howard Tatum, in the
The Dalkon Shield (which had a multi-filiment string) was introduced in the
Second-generation copper-T IUDs were also introduced in the 1970s. These devices had higher surface areas of copper, and for the first time consistently achieved effectiveness rates of greater than 99%. Worldwide today, with the exception of the new GyneFix, this is the only type of IUD available.[10]
Advancements in Health:
Release Date: May 28, 2003
SCHOOL CONDOM AVAILABILITY DOES NOT INCREASE SEXUAL ACTIVITY
By Ann Quigley, Contributing Writer Health Behavior News Service
Making condoms available in high schools does not increase adolescent sexual activity, but it protects those who are already sexually active from some sexually transmitted diseases, according to a survey of more than 4,000 adolescents attending
“Condom availability was not associated with greater sexual activity among adolescents but was associated with greater condom use among those who were already sexually active, a highly positive result,â€ﾝ says Susan M. Blake Ph.D., of the Department of Prevention and Community Health at George Washington University School of Public Health and Health Services in Washington, D.C.
“When condoms are available in schools and are successfully used by sexually active adolescents, they may be an effective means of preventing potentially harmful outcomes such as HIV/STDs and pregnancy,â€ﾝ she adds.
Approximately half of all adolescents in grades nine through 12 report that they have had sex, with nearly 60 percent using condoms during their most recent sexual encounter, according to research cited in the study.
Blake and colleagues analyzed sexual risk behavior data from the 1995
The practice of making condoms available in schools is fairly controversial and many of the small fraction of schools nationwide that do so are in
The researchers found adolescents enrolled in schools with condom availability were less likely to report being sexually active or to report having recent sexual intercourse. Also, sexually active adolescents in these schools were more likely to report having used condoms during their most recent sexual encounter. The study results are published in the June 2003 issue of the American Journal of Public Health.
Blake and colleagues found no pregnancy rate differences between adolescents in schools with and without condom availability programs, possibly because sexually active students in schools without condoms available were more likely to use non-condom contraception. Twenty-five percent of sexually active students in schools without condoms available and 13 percent of students in schools where condoms were made available used other forms of contraception.
“We suspect that the benefits of increased condom use may have been offset by use of other contraceptives among students enrolled in the schools without condom availability programs,â€ﾝ Blake says.
The researchers also found no differences in perceived access to condoms between those who could get condoms at school and those who couldn’t. Students may have been intimidated by the process of accessing condoms through school personnel rather than via a drug store or vending machine. Perhaps “because accessing condoms was likely to result in embarrassment, students did not perceive that access to condoms was greater,â€ﾝ Blake says.
But the finding remains that reported condom use was greater among sexually active teens in condom availability schools. Even if these teens were too embarrassed to get condoms at the school clinic, the program may have indirectly influenced them to use condoms:
“It may not have been making condoms available per se that was associated with greater rates of condom use,â€ﾝ Blake says, “but rather the fact that the adoption of such programs reflected broader community mores, communicated positive social norms and environmental supports and facilitated communication of family values and norms promoting condom use.â€ﾝ
The researchers note the cross-sectional “moment-in-timeâ€ﾝ nature of their study as a limitation. Because the study was not designed to examine changes in condom use from pre- to post-program, they say, its findings don’t definitively prove the condom availability programs influenced teen sexual behaviors.
“Nonetheless, our results suggest that making condoms available, a clear indication of social and environmental support for condom use, may improve HIV prevention practices,â€ﾝ Blake concludes.
“There is a continuing need for effective HIV, STD and pregnancy prevention programs that discourage early onset of sexual activity and encourage protection among adolescents who are already sexually active.â€ﾝ
This study was conducted in collaboration with the Massachusetts Department of Education as part of an evaluation of its HIV/AIDS prevention program
ISSUE AND DEBATE; CONTRACEPTIVES' PRESENCE IN SCHOOLS CREATES A DISPUTE AMONG EDUCATORS
| |
By JANE PERLEZ
Published: October 13, 1986
At nine
Clinics at two of the schools dispensed contraceptive devices to students who had parental permission to use the health services; at the other seven schools, prescriptions for contraceptives were written and the students were referred to the affiliated hospitals and clinics to have them filled.
The health clinics, run by major hospitals or community health centers, were approved by the Board of Education in 1984. But at the time, members were not informed that the dispensing of birth-control devices and prescriptions would be among the services.
The availability of contraceptives, revealed in a report to the board 10 days ago, came as a surprise to board members with the exception of the president, Robert F. Wagner Jr., who said he had learned of it earlier.
To Mr. Wagner, Schools Chancellor Nathan Quinones and Mayor Koch, the prescriptions and dispensing of birth-control devices was not a shock. ''Anyone who believes comprehensive health care for sexually active adolescents does not include birth control is crazy,'' Mr. Wagner said.
There was considerable disagreement among the seven board members. Two said they opposed both prescription writing and distribution of contraceptives. Two others indicated they were opposed to distribution. Only three, including Mr. Wagner, fully supported the policy.
One board member, Dr. Gwendolyn C. Baker, said she believed the distribution of contraceptives encouraged promiscuity, a view that was echoed by the Roman Catholic Archdiocese of New York City and the Diocese of Brooklyn. The Background
In an effort to stem the rise of teen-age pregnancies around the nation, health clinics that provide family planning counseling have been rapidly increasing in public schools.
The Center for Population Options, a private nonprofit group based in
In
A report this year by researchers at the Johns Hopkins School of Medicine showed that counseling and birth-control services to students at two health clinics at public schools in
With these statistics and the general need for student health services in mind, the New York State Department of Health made $1.5 million available for establishment of the nine school health clinics in New York City.
The nine schools and their health providers are: Andrew Jackson in Queens (Jamaica Hospital); William H. Taft in the Bronx (Bronx Lebanon Hospital); Boys and Girls in Brooklyn (Bedford-Stuyvesant Family Health Center); Louis D. Brandeis and Martin Luther King Jr., (St. Luke's Hospital); Manhattan Center for Mathematics and Science, and Julia Richman (Mount Sinai Medical Center), and West Side and Park East (Ryan Community Health Center) in Manhattan. For Contraceptives
Mayor Koch, the United Federation of Teachers, the New York City Council of Churches, an array of children's advocacy groups, as well as Mr. Quinones and Mr. Wagner, vigorously support dispensing of birth-control devices at school clinics, although most add the caveat that parental permission should be obtained.
Search Your Love:
Sex is often a taboo topic in our society. Everyone knows about it, and most people do it, but few are comfortable discussing it. Sometimes, it's even too hard for parents to discuss with their children, so they rely on the schools and education system to do so. Leaving it up to them can result in your child being taught things you didn't necessarily want. Various religious and cultural attitudes exist about sex and the use of contraceptives, and what your school teaches may differ from your personal beliefs.
Certain religious practices believe in preaching abstinence to teens and young adulthoods. Having sex before you're married is considered to be a sin. They are against making condoms available in schools for two main reasons. They believe that sex is an act that occurs between a married man and woman for the sake of reproduction. Contraceptives are not necessary, as no one other than married people should have sex. More liberal religious attitudes are accepting of those who have sex out of wedlock and support the use of contraceptives in these situations. However, they believe that making them available to teenagers will just encourage them to be sexually active.
The other side of the debate wants to hand out condoms in schools. Although some do believe in the religious practices that teach only abstinence, they are realistic about the increase of sexual activity among teens and even pre-teens. They believe the teens are going to have sex whether or not you provide them with any contraceptives, so you might as well make condoms available to them.
Studies have shown though that the best way to get kids to wait to be sexually active until their older and to use contraceptives is a school program that combines a multi-facet sex education program along with making condoms available in school. Research has shown that these kids will wait longer than their peers to have sex, and they will be more informed about pregnancy and birth control. They will have a better grasp of the consequences of their actions and understand the importance of why they need to use contraceptives when they decide to be sexually active.
If you are concerned with what a school teaches your child in sex education, you should remember that their primary educator, even surpassing their school is you their parent. The views and attitudes you have regarding sex may differ from the schools. It is your responsibility to discuss sex, pregnancy and birth control with your child, especially if you disagree with the school.
No one knows for sure if making contraceptives available in schools just encourages teens to be sexually active or meets the demand of what is already occurring across the country. Therefore, it's your job as a parent to educate your child and make a decision regarding how you're going to prepare them.
If you choose to make contraceptives available to your child, it is your responsibility to educate your child about them and pregnancy and birth control. Teens must realize there can be serious consequences if they decide to be sexually active.
When a man and a woman have sexual intercourse and do not want the woman to become pregnant, they use birth control, or contraception, to try to prevent pregnancy. Deciding when to have a child and how much time should pass between the birth of one child and the birth of the next is important. It allows parents to have children when they are ready to love and care for them and be responsible for their many needs.
Methods of Birth Control
To understand how birth control works, you must understand some basic facts about human reproduction. About once every 28 days an egg cell, or ovum, is released from one of a woman's ovaries and begins to travel down a narrow fallopian tube toward the uterus. This is called ovulation. If this ovum is not fertilized by a sperm cell from a man, it will quickly die and pass out of the uterus during a normal menstrual period. But if sexual intercourse takes place while the ovum is in the fallopian tube, one of millions of sperm cells deposited in the woman's vagina by the man's penis may unite with the ovum. This is called conception--fertilization of the ovum. The fertilized egg may then attach itself to the inner lining of the woman's uterus, and a new life will begin to develop--the woman is pregnant. Some nine months later a baby will be born.
Barrier Methods.
Barrier methods of birth control prevent pregnancy by blocking sperm from reaching the ovum so conception cannot take place. The condom is a sheath of thin latex rubber that is rolled down over the penis before intercourse to trap ejaculated sperm. A diaphragm is a dome of thin rubber that is inserted into the vagina before sex. It covers the opening of the uterus, blocking the entrance of any sperm. A condom for women was introduced in the 1990's. It looks similar to a diaphragm with a long polyurethane sheath attached. The vaginal sponge and the cervical cap can be inserted into the vagina; they function much like a diaphragm. Barrier methods of birth control are best used with a spermicide--a chemical that kills sperm cells. Condoms can be bought at a pharmacy; diaphragms and cervical caps must be fitted by a doctor. The vaginal sponge is not currently available in the
Hormonal Methods.
Hormonal methods of birth control work by changing a woman's body chemistry so that eggs are not released from the ovaries. Some forms also make it hard for any fertilized egg cell to attach itself to the uterus lining and grow. In the
Hormonal methods of birth control must be prescribed or administered by a doctor, who can discuss and monitor any side effects that may occur. When used properly, they are about 99 percent effective. Fertility returns shortly after the hormones are stopped.
Intrauterine Device (IUD).
An intrauterine device is a small coil of metal or plastic that is inserted by a doctor into the woman's uterus. It prevents any fertilized egg cell from attaching itself in the uterus. Younger women who use IUD's are sometimes bothered by infections, cramping, or unusual menstrual bleeding. IUD's are used more easily by older women who have already had children. They are over 97 percent effective for preventing pregnancy.
Natural Birth Control.
Natural birth control methods attempt to prevent conception without the use of artificial devices. The Fertility Awareness Method (FAM), or rhythm method, depends upon avoiding sexual intercourse on the days before, during, and after ovulation--about 12 days each month--so that no sperm are present to fertilize the egg cell. It can be difficult to pinpoint the time of ovulation, so the FAM, like another natural birth control technique--withdrawing the penis from the vagina just before sperm are ejaculated--can be unreliable.
Emergency Birth Control.
"Morning after pills" can be used to prevent pregnancy after intercourse when a birth control method failed or no method was used. The pills contain hormones that delay ovulation or keep a fertilized egg from attaching to the uterine wall. The pills are available from a doctor. They are about 75 percent effective. Side effects include nausea and vomiting.
Sterilization.
Sterilization--surgically cutting and tying off or sealing the woman's fallopian tubes (tubal ligation) or the man's sperm tubes (vasectomy)--is a way to prevent pregnancies permanently. These operations can seldom be reversed.
History of Birth Control
Birth control is a fairly recent idea in human history. Centuries ago large families were important for survival. Many babies died in infancy and childhood, and people had large families to be sure that enough children would survive to help gather food and have children of their own.
In the 1700's and 1800's, medical and scientific discoveries enabled many diseases to be controlled and food supplies to be increased. Large families were no longer necessary for survival. People lived longer, and the world's population began to grow rapidly.
In 1798 Thomas Malthus, an English economist, predicted that the world's population would quickly outgrow the available food supply. He recommended people stop having so many children.
Malthus' prediction proved to be wrong. But as the world population increased rapidly in the early 1900's, the idea of family planning took root. Methods of contraception began to be developed, and a worldwide movement to encourage birth control began.
Margaret Sanger, a trained nurse, became a leader of that movement in the early 1900's. She published pamphlets and books about family planning and helped pass laws that allowed doctors to teach birth control methods. Sanger also established clinics to advise and educate people and founded the National Birth Control League. These were combined to form the Planned Parenthood Federation of America in 1942.
Birth Control Today
The world now has more than 6 billion people, and the number is increasing rapidly, especially in developing countries. Different countries support birth control in different ways. In Europe and the
Not everyone favors the use of birth control, however. The Roman Catholic Church forbids artificial methods of birth control, believing that sexual love in marriage should never be separated from the possibility of conception. Only abstinence (refraining from sexual intercourse) or the Fertility Awareness Method of family planning are considered acceptable. Some people fear that governments may impose birth control in order to obtain political control over their people. Others charge that teaching birth control encourages people to have sex before marriage--an idea that is not supported by scientific evidence.
Doctors and other knowledgeable people you respect can help you understand the different methods of birth control and how they may affect your life and your health.
Alan E. Nourse, M.D.
Author, Birth Control and Teen Guide to Birth Control
n the spring, poppy seeds sprout. Poppy plants burst into leaf. They flower and produce seeds. In the fall, with the coming of frost, the poppies die. But their seeds survive and sprout the following spring. A new generation of poppies replaces the one that has died. Poppy plants continue to exist.
Every kind of living creature, from tiny bacteria to enormous whales, makes new living things like itself. In other words, it reproduces. An individual plant or animal may fail to reproduce, but that does not hurt the individual. Nor does it hurt the species (kind) of animal or plant, so long as enough other individuals of that kind reproduce.
Among living creatures there are two main types of reproduction. In one type the new individuals, or young, come from a single parent. Bacteria, for example, grow to a certain size and then divide into two equal parts. Each new bacterium is a small copy of the parent. This type of reproduction is called asexual reproduction.
The other main type is called sexual reproduction. In this type, young are produced by two parents or from two different parts of the same parent. When two deer mate, for example, living material from both animals is joined, or fused. In time a fawn is born, looking like its parents but not exactly like either one.
Some living things can reproduce sexually at some times and asexually at others.
Asexual Reproduction
There are three main ways that living things reproduce asexually. They may divide like bacteria, they may form buds, or they may create spores. The simplest of these ways is dividing.
Dividing
Bacteria, amoebas, and many other tiny one-celled creatures reproduce by dividing in half. Some larger creatures, too, may divide in half. This is true, for example, of a freshwater relative of the earthworm. The body of this worm slowly pinches in around the middle. Finally the body separates in two. The front end grows a hind end, while the hind end grows a front end.
New plants may grow from pieces of an older plant. For example, if you set a sweet-potato root (the part you eat) in water, the root will sprout leafy shoots and new roots. In time you will have a sweet-potato vine.
Budding
A number of living things, such as yeasts, sponges, corals, and grasses, can reproduce by forming buds. Unlike flower or leaf buds, these buds develop into complete new individuals. The threadlike hydra will serve as an example. This is a tiny water animal related to corals and jellyfish.
Sometimes a hydra develops a bulge on its side. This bulge--which is a bud--slowly grows into a complete new animal just like the parent. The new hydra may separate from the parent and take up life on its own. Or it may remain attached to the parent and, in turn, produce buds. In some creatures entire colonies, or groups, may form this way, each individual a descendant of the one original parent.
Spores
Some plants reproduce by means of tiny specks of life called spores. You may have seen tiny black dots on the undersides of fern leaves. These dots are spore cases filled with spores. When the spores are ripe, they fall. Those that land on moist, cool ground sprout into tiny new plants.
Mosses and a number of other plants reproduce by forming spores. So do yeast cells and certain one-celled creatures.
Sexual Reproduction
Most creatures, even those that can reproduce asexually, reproduce by sexual means. For example, let us look again at the tiny hydra. A bud develops on the parent hydra. This time the bud does not develop into a complete new individual. This bud contains sperm if the hydra is a male or eggs if the hydra is a female. Eggs and sperm are special reproductive cells.
When the eggs or sperm are ripe, the bud bursts open and sheds its contents into the water. The eggs, as in all creatures, are bigger and rounder than the sperm. Eggs contain a food supply that will nourish the new individual as it develops. Unlike sperm, eggs have no means of moving about.
Hydra sperm, like those of most species, have thin, whiplike tails that enable them to swim toward an egg. A sperm is very much smaller than an egg. It lacks a food supply of its own.
Hydras produce many eggs or sperm in a single bud. Some of the sperm from the male hydras meet some of the eggs from the females. When a sperm fuses with an egg, the egg is said to be fertilized. A fertilized egg contains living material from two parents. It grows and develops into a new individual combining the traits of both parents.
Sexual reproduction in most creatures is more complicated than in the hydra. However, it involves the same basic steps. First, reproductive cells must form. In most species the two sexes are separate. Eggs are formed by females, and sperm are formed by males. Next, eggs must be fertilized. Sperm need a fluid in which to swim to the egg. If sperm are not shed into water, the body must produce the necessary fluid. Finally, some sort of food and protection for the developing egg must be provided until the young can care for itself.
Care of Eggs Shed in Water
Hydras and many other animals shed their eggs and sperm directly into water. They do little if anything to ensure the survival of their offspring.
Many other animals have special ways of making sure that their eggs and sperm meet and that their fertilized eggs are protected.
A female trout, for example, makes a nest. She uses her tail fin to scrape a hollow in a stream bottom. There she sheds her eggs. The male swims along beside her and covers the eggs with sperm. The female then scrapes gravel over the fertilized eggs. This helps prevent enemies from eating the eggs.
The eggs of some water animals are protected with a thick jellylike coating. In such species the sperm fertilizes the eggs just as they leave the female's body and before the coating is formed. A male horseshoe crab, for example, clasps the female's shell and sheds his sperm directly over the eggs. The pair usually dig into the sand before depositing their eggs and sperm. As the mating pair withdraws, the sand drifts over the eggs, giving them extra protection.
In many lobsters, crabs, and shrimps, the male clasps the female and fertilizes the eggs as they are shed into the water. Frogs and toads mate in a similar fashion.
Fertilization Inside the Body
In salamanders, which are related to frogs and toads, eggs are fertilized inside the female. The male courts his mate and then deposits one or more jellylike packets of sperm on the ground or the pond bottom. The female squats over a packet and draws it up into an opening beneath her tail. The sperm fertilize the eggs before the eggs are shed.
Male squids also produce packets of sperm. A male uses a special tip on one of his tentacles to place the packet inside the body covering of the female.
Snails, guppies, sharks, rays, some crabs, and other water animals also fertilize their eggs internally. In some the eggs remain inside the female until they hatch.
Among land animals, fertilization usually occurs internally. If eggs and sperm were shed directly onto the ground, they would soon dry out in the air. Moreover, the sperm would have no way of swimming toward the eggs unless dew or rainwater happened to be present.
Male spiders deposit their sperm in a drop of fluid. Then they place the drop of fluid in an opening on the body of the female. She later spins a cocoon in which she lays the fertilized eggs.
In most land creatures, however, fluid containing sperm is inserted directly from the body of the male into the body of the female. This is true of insects, reptiles, birds, and mammals, including man.
In seed plants, such as pine trees and daffodils, both eggs and sperm have heavy coatings that keep them from drying out. The sperm, contained in tiny pollen grains, may be transferred by wind or by insects to the female part of the flower. Although the sperm have no tails, they are carried down into the egg in a special pollen tube. The fertilized egg is protected inside a tough seed coat and by the fruit.
Care of Eggs Fertilized Internally
Most insects lay their eggs in protected places where the newly hatched insects will find food. Insect eggs are very small and contain very little food. Insects hatch before they are fully developed, and most species pass through several stages before they look like their parents.
The eggs of both birds and reptiles are well supplied with yolk and protected by shells. With a large yolky egg the young develop into small copies of their parents before food is needed from outside.
Snakes, lizards, and turtles lay their eggs in holes in the ground, and many cover the nests with mud or sand. The eggs of some lizards and snakes remain inside the mother until the young hatch.
Most birds build nests of some sort and hatch the eggs with the warmth of their own bodies. The parents take turns feeding and guarding the young.
In most mammals the eggs are small and have little yolk. The developing individual gets the nourishment it needs from its mother's body. After birth the young suckle milk from the mother. The result is that the offspring are large before they have to fend for themselves.
Small mammals, such as mice and foxes, may have from two to a dozen or more offspring at a time. Few of these offspring manage to grow up. Large mammals, such as horses and elephants, usually have only one offspring at a time. The parents feed and guard the young for a long time. Each offspring has a good chance of growing to full size and of reproducing in turn.
Human beings usually produce babies one at a time. Their babies are helpless at birth. But they are so well taken care of that each has a better chance of growing up than any other kind of creature.
See also: Cells; Eggs and Embryos; Flowers; Genetics; Metamorphosis.
How to cite this article:
MLA (Modern Language Association) style:
Berrill, N. J. "Reproduction." The New Book of Knowledge®. 2007. Grolier Online. 9 Apr. 2007
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