multiple birth, bringing forth of more than one offspring at birth. Although many smaller mammals bear several young at a time, multiple births are relatively uncommon in humans and other primates. Twinning, the process that leads to the production of more than one offspring, results in twins, and with decreasing frequency, triplets, quadruplets, quintuplets, sextuplets, septuplets, and octuplets.

In the one-egg, or identical, type of twinning, a single fertilized ovum divides to form two complete organisms. Such twins are always of the same sex, are usually extraordinarily similar in physical appearance, and have identical blood-group types. Twinning to form one-egg identical twins usually takes place early in pregnancy. If considerable development has taken place before the twinning occurs, there may be an incomplete separation of the two embryos resulting in conjoined offspring called Siamese twins.

Fraternal twins, which are more common than identical twins, are those that develop from two separate ova, each fertilized by a sperm. Fraternal twins may or may not be of the same sex and need not resemble each other more than do any other two offspring of the same parents. In the United States twins occur once in approximately 40 births. In rare cases, non-identical embryos can fuse in the womb to produce a condition called chimerism, in which some of the individual's cells come from one of the embryos and the rest of the cells come from the other, genetically distinct embryo.

The incidence of multiple-egg births is in part genetically determined, varying according to race and family tendencies; and it is also influenced by external factors, i.e., the incidence increases with increasing age of the mother and the number of children she has already borne. One-egg, or identical, twinning occurs with the same frequency in all women, regardless of race, age, or other factors. There is evidence from comparative biology that deleterious factors in the environment of the newly fertilized ovum, such as a reduction in oxygen, increase the likelihood of one-egg twinning. Fertility drugs such as clomiphene, which are used when the cause of infertility is lack of released ova, sometimes cause several ova to be released and fertilized simultaneously. The use of these drugs has led to a rise in the incidence of multiple births, including sextuplets, septuplets, and octuplets.

birth rate: see vital statistics.

birth defects, abnormalities in physical or mental structure or function that are present at birth. They range from minor to seriously deforming or life-threatening. A major defect of some type occurs in approximately 3% of all births. Defects may be genetic in origin, as in Down syndrome, Tay-Sachs disease, sickle cell disease, and hemophilia, or may be the result of infections, such as rubella and sexually transmitted diseases. Other teratogenic (malformation-causing) agents include drugs or hormones taken by the mother (e.g., thalidomide and DES) and maternal illnesses (e.g., diabetes). The mother's nutrition, drinking (see fetal alcohol syndrome), smoking, and drug abuse, as well as exposure to toxic chemicals and radiation, can also affect the developing fetus. Smoking, drugs, toxic chemicals, and the like can also damage the father's sperm, which may pass on the defect to the embryo in fertilization. The incidence of some disorders is elevated when the mother or father is older, which increases the likelihood of age-related gene mutations. Certain birth defects can now be detected prenatally through amniocentesis and chorionic villus sampling. Surgical procedures to correct certain disorders before birth are still considered experimental.

birth control, practice of contraception for the purpose of limiting reproduction.

Methods of Birth Control

Male birth control methods include withdrawal of the male before ejaculation (the oldest contraceptive technique) and use of the condom, a rubber sheath covering the penis. The condom, because of its use as a protection against sexually transmitted diseases, including AIDS, has become a frequently used birth control device.

Contraceptive methods for women include the rhythm method—abstinence around the most likely time of ovulation—and precoital insertion into the vagina of substances (creams, foams, jellies, or suppositories) containing spermicidal chemicals. The use of a diaphragm, a rubber cup-shaped device inserted before intercourse, prevents sperm from reaching the uterine cervix; it is usually used with a spermicide. Contraceptive sponges, which are impregnated with a spermicide, also are inserted into the vagina before intercourse and work primarily by acting as a barrier to the sperm. Intrauterine devices, or IUDs, are variously shaped small objects inserted by a doctor into the uterus; they apparently act by creating a uterine environment hostile either to sperm or to the fertilized egg.

The birth control pill, an oral contraceptive, involves a hormonal method in which estrogen and progestins (progesteronelike substances) are taken cyclically for 21 or 84 days, followed by 7 days of inactive or no pills. The elevated levels of hormones in the blood suppress production of the pituitary hormones (luteinizing hormone and follicle-stimulating hormone) that would ordinarily cause ovulation. An oral contraceptive formulation that utilizes no inactive pills and is taken every day (and completely suppresses menstruation) also exists. Estrogen and progestins may also be delivered through the weekly use of a contraceptive skin patch or the monthly use of a vaginal ring (a flexible plastic ring inserted in the vagina); both slowly release the hormones they contain.

Sterilization of the female, often but not always performed during a Cesarean section or shortly after childbirth, consists of cutting or tying both Fallopian tubes, the vessels that carry the egg cells from the ovaries to the uterus. In male sterilization (vasectomy) the vas deferens, the tubes that carry sperm from the testes to the penis, are interrupted. Sterilization, in most cases irreversible, involves no loss of libido or capacity for sex.

No contraceptive yet devised is at once simple, acceptable, safe, effective, and reversible. Some, such as the diaphragm, condom, and chemical and rhythm methods, require high motivation by users; the pill, which must be taken daily, sometimes induces undesirable side effects, such as nausea, headache, weight gain, and increased tendency to develop blood clots. The IUDs, although requiring no personal effort or motivation, are often not tolerated or are expelled, and they sometimes, particularly if poorly designed, cause uterine infection, septic abortion, and other problems.

If birth control fails (or is not used), doctors may prescribe several large doses of certain oral contraceptives as "morning after" pills; the high level of hormones can inhibit the establishment of pregnancy even if fertilization has taken place. Mifepristone, or RU-486, the so-called abortion pill, is effective within seven weeks after conception and requires close medical supervision. It was first approved in Europe and was tested in the mid-1990s in United States, where it was approved in 2000. Another experimental technique is immunization against human chorionic gonadotropin (HCG), a hormone secreted by a developing fertilized egg that stimulates production of progesterone by the ovary; the effect of the anti-HCG antibody would be to inactivate HCG and thereby induce menstruation even if fertilization occurred.

See also abortion; menstruation; reproductive system.

History of the Birth Control Movement

Although contraceptive techniques had been known in ancient Egypt, Greece, and Rome, the modern movement for birth control began in Great Britain, where the writings of Thomas Robert Malthus stirred interest in the problem of overpopulation. By the 1870s a wide variety of birth control devices were available in English and American pharmacies, including rubber condoms and diaphragms, chemical suppositories, vaginal sponges, and medicated tampons. Easy public access to contraceptive devices in the United States aroused the ire of Anthony Comstock and others, who lobbied Congress until it passed (1873) a bill prohibiting the distribution of these devices across state lines or through the mail. Moreover, in England in 1877, Annie Besant and Charles Bradlaugh were tried for selling The Fruits of Philosophy, a pamphlet on contraceptive methods, written in 1832 by an American, Charles Knowlton. After their famous trial, the Malthusian League was founded. Meanwhile, a variety of contraceptive devices remained available to a large public, usually advertised in veiled but unmistakable language.

In 1878 the first birth control clinic was founded in Amsterdam by Aletta Jacobs. The first U.S. birth control clinic, opened (1916) by Margaret Sanger in Brooklyn, N.Y., was closed by the police; she received a 30-day jail sentence. She later permanently established a clinic in New York City in 1923. In Great Britain the Malthusian League, aided by Marie Stopes, established a birth control clinic in London in 1921.

Sanger also helped organize (1917) the National Birth Control League in the United States; in 1921 it became the American Birth Control League, and in 1942 the Planned Parenthood Federation of America. Meanwhile, in 1918 an American judge ruled that contraceptive devices were legal as instruments for the prevention of disease, and the federal law prohibiting dissemination of contraceptive information through the mails was modified in 1936. Throughout the 1940s and 50s, birth control advocates were engaged in numerous legal suits. In 1965 the U.S. Supreme Court struck down the one remaining state law (in Connecticut) prohibiting the use of contraceptives.

The federal government began to take a more active part in the birth control movement in 1967, when 6% of the funds allotted to the Child Health Act was set aside for family planning; in 1970, the Family Planning Services and Population Act established separate funds for birth control. Birth control and sex education in schools continue to be emotional issues in the United States, where adolescent sexual activity and pregnancy rates are high and bring with them increased risks of sexually transmitted diseases and complications of pregnancy, as well as societal and personal costs.

Birth control on the international level is led by the International Planned Parenthood Federation, founded in 1952, with members in 134 countries by 1995. Sweden was one of the first countries to provide government assistance for birth control, which it did as early as the 1930s. Two of the more successful birth control programs have been in Japan, where the birthrate has been dramatically reduced, and—more controversially—in China, where the government has a "one family, one child" policy and local authorities have typically intimated women pregnant into aborting a second pregnancy. Several of the so-called underpopulated nations, however, have a stated policy of encouraging an increased birthrate, e.g., Argentina, and concern over declining populations has increased in recent years in certain Western European countries and Russia. Among religious bodies, the Roman Catholic Church has provided the main opposition to the birth control movement; popes Paul VI and John Paul II reaffirmed this stance in encyclicals.

birth or labor, delivery of the fetus by the viviparous mammal. Birth is also known as parturition. Human birth normally occurs about 280 days after onset of the last menstrual period before conception.

The Stages of Labor

Onset of labor, the first stage, is heralded by contractions of the uterus felt as cramplike pains in the abdomen or lower back that recur at intervals of 10 to 30 minutes and last about 40 seconds; they increase in frequency until they occur at about 2-minute intervals. With each contraction the cervix, or neck of the uterus, dilates until it becomes wide enough, about 4 in. (10 cm), to permit emergence of the baby.

In the second stage of labor the baby passes through the birth canal, most commonly head first, and is born. The effectiveness of uterine contractions in this stage is enhanced by the bearing-down abdominal contractions of the mother.

The third stage of labor, which occurs about 15 to 30 minutes after the child is born, is characterized by the separation of the placenta from the uterine wall and its expulsion. The total time of labor averages 13 to 14 hours in women pregnant for the first time and 8 to 9 hours in women who have previously borne children.

Methods of Analgesia

The pain of childbirth can be relieved with a variety of analgesic and sedative drugs, including morphine, barbiturates, and chloroform. However, many drugs that relieve pain also slow the uterine contractions or dangerously depress the baby's respiratory system. Spinal anesthetics, injected directly into the spinal cord, while not dangerous to the child, are difficult to administer accurately and are therefore potentially dangerous to the mother. Hypnosis has also been used experimentally.

Natural Childbirth

In recent years so-called natural childbirth has come into wide use; the advantages are that the child is born undrugged and the mother can be conscious at the moment of birth. Natural childbirth emphasizes the ability of many women to give birth with a minimal amount of pain-killing drugs or none at all. The Dick-Read method, formulated by the British obstetrician of that name, emphasizes maternal understanding of the birth process as an aid to relaxation, and exercises to strengthen muscles and encourage proper breathing. The Lamaze method, or psychoprophylaxis, is of Russian origin; it uses breathing exercises as a conditioned response to uterine contractions.

Complications of Childbirth

Birth often cannot proceed normally because of a defect of the cervix or weak uterine contractions; breech births, in which the feet or buttocks emerge first, and transverse births, in which the child is positioned across the uterus, usually require obstetrical intervention, such as forceps delivery, manually turning the baby, or performing a cesarean section. About 10% of pregnancies terminate in deliveries that are too early, producing (after at least 200 days of gestation) premature infants requiring special care. Birth of a fetus prior to about 200 days of gestation is termed a miscarriage; birth within the first three months, an abortion. Stillbirth is the delivery of a dead child.

Complications of childbirth affecting the newborn include infant blindness attributable to gonorrhea infection, now largely eliminated by routine administration of silver nitrate to the eyes; retrolental fibroplasia, a type of blindness common for some years in premature infants that was found to result from administration of high concentrations of oxygen and is now largely avoided; and erythroblastosis fetalis, or Rh disease, which can often be prevented. Puerperal fever, an infection of the mother's genital tract once common following labor and delivery, has now also been largely eliminated by preventive hygiene, especially in labor, and by antibiotic therapy.

See pregnancy; obstetrics.

Birth less than 37 weeks after conception. Infants born as early as 23–24 weeks may survive but many face lifelong disabilities (e.g., cerebral palsy, blindness, deafness). Premature infants account for 8–9percnt of live births but two-thirds of infant deaths. 40–50percnt of cases have no explanation; other cases can be attributed to such causes as maternal hypertension or diabetes, multiple pregnancy, or placental separation. With good care, about 85percnt of live-born premature infants should survive. Infants born very early (before 32–34 weeks) lack fully developed lungs and often develop respiratory distress syndrome. They also have problems maintaining body temperature and fighting infection. Most deaths result from breathing problems, infections, and brain or lung hemorrhages. Premature infants are characterized by low birth weight, small size, irregular breathing, absence of subcutaneous fat, and thin skin.

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Birth of more than one child from one pregnancy. Twins are most common, born in 1 of about every 80 pregnancies. Identical twins develop from a single fertilized egg, which splits into two genetically identical embryos (though physical traits may be modified during their development); they occur randomly but are more likely in older mothers. Incomplete or late division results in conjoined twins. Fraternal twins develop from two eggs fertilized by two sperm and are no more genetically alike than are other siblings. Most common among persons of African ancestry and least common among those of Asian ancestry, fraternal twins seem to run in families. Repeated twinning produces triplets, quadruplets, and so on; these multiples may be identical, fraternal, or a combination. The use of fertility drugs has increased the number of high-order multiple births. Medical and psychological “twin studies” compare fraternal and identical twins to learn about genetic influences on various characteristics and diseases.

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A large group of disorders arising during development that cause abnormality of the human body. Most are due to genetic factors such as inherited or spontaneous mutations, whereas others are caused by environmental influences during pregnancy such as exposure to harmful chemicals. The most severe and lethal disorders arise during the first 28 days of development and include gross brain anomalies and heart defects. The mildest malformations occur in the late stages of development and are often the result of dominant inheritance, whereas complex congenital syndromes are often the result of recessive inheritance. Seealso birth defect; Down syndrome.

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Voluntary limiting of human reproduction, using such means as contraception, sexual abstinence, surgical sterilization, and induced abortion. The term was coined in 1914–15 by Margaret Sanger. Medically, birth control is often advised when childbirth might endanger the mother’s health or substantial risk exists of bearing a severely disabled child. Socially and economically, limitation of reproduction frequently reflects a desire to maintain or improve family living standards. Most religious leaders now generally agree that some form of fertility regulation is desirable, though the means are strongly debated. Seealso family planning.

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or birth or childbirth or labour or delivery

Process of bringing forth a child from the uterus, ending pregnancy. It has three stages. In dilation, uterine contractions lasting about 40 seconds begin 20–30 minutes apart and progress to severe labour pains about every 3 minutes. The opening of the cervix widens as contractions push the fetus. Dilation averages 13–14 hours in first-time mothers, less if a woman has had previous babies. When the cervix dilates fully, expulsion begins. The “water” (amniotic sac) breaks (if it has not already), and the woman may actively push. Expulsion lasts 1–2 hours or less. Normally, the baby's head emerges first; other positions make birth more difficult and risky. In the third stage, the placenta is expelled, usually within 15 minutes. Within six to eight weeks, the mother's reproductive system returns to nearly the prepregnancy state. Seealso cesarean section; lactation; midwifery; miscarriage; natural childbirth; obstetrics and gynecology; premature birth.

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