Multiple births

Biology

Anatomy or system affected: All

Specialties and related fields: Embryology, genetics, neonatology, obstetrics, pediatrics

Definition: The presence of two or more fetuses in the womb.

Introduction

Multiple births have historically been rare events, but the incidence is increasing with assisted reproductive technologies and improvements in prenatal care. The most common multiple births are twins. In the United States, the twin birth rate is approximately 33.9 per 1,000 live births. Twins can come from a single egg or from two different eggs. The triplet or higher order birth rate is approximately 113.5 per 100,000 live births.

As the number of fetuses increases, the chances that all will survive decreases. Multiple births are most commonly combinations of twins and single eggs. By reviewing the mechanics of twin formation, greater multiples can be understood.

The Different Types of Twins

Two types of twins are well known: fraternal twins and identical twins. Behind these general terms, however, lies considerable variation. This variation is based on the many changes that a human ovum can undergo after it is released by the ovary, is fertilized and becomes a zygote, travels along the Fallopian tube to the uterus, and implants there to develop into an embryo.

Fraternal twins are also known as dizygotic twins. In a normal menstrual cycle, only a single egg is released. When a sperm penetrates an ovum, the fertilized egg releases a chemical that prevents other sperm from penetrating the same egg. If a second egg has been released, however, it can also be fertilized. A newly fertilized egg is called a zygote. If both zygotes succeed in attaching to the uterine walls, a twin pregnancy begins. Usually, this dual insemination occurs during a single release of semen in a single copulation, so that the embryos have the same father. Occasionally, the two eggs may be fertilized in separate copulations during the same ovulation, a phenomenon called superfecundation. It is then possible for dizygotic twins to have different fathers. This possibility seems to have long been recognized. The Greek myth of Leda and the Swan derives from such a pregnancy.

Fraternal twins have separate placentas and membranes in the womb. The placenta comprises maternal and fetal tissues interconnected by blood vessels. Nutrients pass from the mother to the fetus through the placenta. Waste products are removed from the fetus by a reverse process. Sometimes, the placentas press against each other in the womb and fuse. Having had separate placentas or one fused together, however, does not affect the nature of the twins after birth. Fraternal twins, even though they share the same birthday, are no more similar in appearance or manner than two siblings from separate births.

Identical twins are the result of different initial events. They are also called monozygotic or monovular twins. Identical or look-alike twins originate when a single egg spontaneously divides after penetration by a sperm cell. Each half develops separately with the same genetic information. The reason for this division is not known. One theory holds that sometimes the fertilized ovum does not implant in the uterus right away as is normally the case. During the delay, the chromosomes double and the zygote halves, with each half then implanting and becoming a separate embryo.

Variation sometimes appears after birth in monozygotic siblings. Twins can vary in birth weight greatly (one may weigh twice as much), develop at different rates, and die from unrelated natural causes. As a rule, however, identical twins share an overwhelming majority of traits. When two (or more) siblings share a trait, they are considered to be concordant for that trait. Typically, body structures and coloration will be strikingly concordant. Features such as facial shape, hair texture and color, eye color, and height are typical examples of concordance. “Mirror” twins account for one-quarter of identical twins. They show mirror-image symmetry in some traits. For example, one may be left-handed while the other is right-handed. Whorls on the scalp may also occur as mirror images. In very rare cases, one mirror twin will have situs inversus: the placement of all internal organs is reversed. An individual with situs inversus will have the liver and appendix on the left side of the abdomen and the spleen on the right.

Genetic variation may account for the subtle variations in even the most concordant of twins. The internal environment of the womb also has an effect. Most identical twin fetuses share the same placenta but have different inner or chorionic sacs. They may have separate placentas (and separate chorions) depending on when the initial splitting of the zygote took place. Usually, those sharing a single placenta have separate chorions. In the rarest variation, the fetuses also share the same amnion. The degree of separation or number of barriers can influence the amount of oxygen or nutrients that each twin receives. Relatively minor differences can affect development.

A third type of twin is theoretically possible. During maturation and before becoming fertilized, the mature ovum could divide into a secondary oocyte (the cell to be fertilized) and a much smaller polar body. It is possible for the ovum to divide into roughly equal portions, both of which are viable and contain the same genetic material. If separate sperm then fertilize these ova, they would become two zygotes. Such twins would have exactly the same maternal genes, but a portion of the paternal genes would differ. They would be less identical than monozygotes but more so than dizygotes. Although this type of twinning has been described in rats and mice, no human case has been indisputably identified and reported.

A fourth variant of twinning is conjoined twins, formerly referred to as Siamese twins. Conjoined twins share some tissue. This can range from simple joining of skin on the head or shoulders to having one heart or kidney or two torsos and a single pair of legs. Conjoined twins are identical (or monozygotic) twins created by incomplete cell division during early fetal life. The portions of cells that divide normally continue to develop in a normal fashion. The cells that did not divide completely also develop normally. The result is a portion of the body that is duplicated and a portion that is not. If the incomplete division occurred early in fetal development, the amount of shared tissue is likely to be greater than with an incomplete division that occurred later in fetal development.

About one-third of all twin births result in identical twins. The proportion of males and females is approximately equal. The incidence of identical twins remains stable throughout the world's diverse ethnic populations. Fraternal twins, however, show different proportions and distributions. About half the pairs have the same gender (with a nearly equal number of male-male and female-female pairs); about half are male-female pairs. Fraternal twin births occur most frequently among rural Nigerians (45 pairs per 1,000 births) and least frequently among Chinese and Japanese parents (4 pairs per 1,000 births).

Evidence suggests that women inherit a tendency to conceive fraternal twins from their mothers. There is little scientific evidence to support the belief that fathers possess a gene for monozygotic twins. Physiological factors can increase the likelihood of a woman having fraternal twins. Women who are tall and who have previously given birth to children have more twins than small women or those who have not been pregnant before. Women between thirty-five and forty years of age are the most likely to have twins, but the chances decrease thereafter.

Naturally occurring multiple pregnancies (triplets and more) are usually combinations of twins. Identical triplets do rarely occur, but triplets consisting of two identical and one fraternal sibling are far more common. Naturally occurring multiple births of four or more infants are almost always combinations of twins. Physicians can ascertain the status of multiple birth siblings by examining placentas and chorionic sacs.

Possible Complications

Multiple births present greater health risks, both for the mother and her children. Multiples are more difficult to carry in the womb and to nurture through infancy than singletons. Women who are pregnant with multiples often need to go to additional prenatal care checkups to monitor the pregnancy for complications. Women who are pregnant with multiples face higher risks of gestational diabetes, gestational hypertension, preeclampsia, miscarriage, and stillbirth. Multiple babies are at a higher risk of premature birth and having a low birth weight. Most physicians recommend birth by cesarean section to manage complications better, particularly a higher risk of breech presentation, in which the fetus's buttocks or feet are positioned closest to the birth canal, rather than the head. Most twins are born healthy, but they must be monitored carefully. As the number of fetuses increases, their size decreases and the risk of other health complications increases. Because they are not fully mature, this increases the chances for medical problems.

Positively identifying multiple fetuses in the womb is not always an easy task, even though medical science has developed a variety of techniques. The traditional signs of considerable fetal movement, multiple heartbeats, and a large weight gain by the mother can be inaccurate and contradictory. Tests for the human chorionic gonadotropin hormone in the mother's blood or urine or alpha-fetoprotein in the blood may suggest the presence of multiple fetuses if the hormone or protein levels are unusually elevated. Nevertheless, imaging technologies provide the most reliable test. Ultrasonography can usually resolve multiple fetuses early in the pregnancy.

A multiple pregnancy itself strains the mother's body and is particularly subject to medical complications. A mother who was a healthy weight before pregnancy and who is carrying multiple fetuses should gain about 37 to 54 pounds, about twice the weight of a single pregnancy. The added weight can cause skeletal and muscular problems. The fetuses' demands on the mother's body may also worsen preexisting medical conditions, such as heart or kidney disease. As the multiple fetuses develop, their size stretches the uterus, which can initiate early labor. For this reason, the premature birth rate is higher for multiple fetuses than for single fetuses. Twins occasionally reach full term; triplets and greater multiples do not.

Similarly, multiple pregnancies miscarry at more than three times the rate of singletons. Occasionally, one fetus will develop at the expense of the other by drawing a disproportionate amount of nutrients from the mother, a condition called twin transfusion syndrome. In cases of identical (monozygotic) twins who share a single placenta, a phenomenon known as a twin-to-twin transfusion syndrome can occur. When this occurs, the blood supply of one twin moves to the other through the shared placenta so that one twin receives most of the blood, nutrients, and oxygen, in turn becoming much larger than the other twin. In some instances, the smaller twin (the donor) perishes due to lack of these vital substances. In other cases, the larger twin (the recipient) succumbs to heart failure as a result of having to pump the increased blood flow. Fetal laser surgery may be undertaken to stop the flow of blood from one twin to the other. If one fetus dies for any reason, then the mother's body may reabsorb it partially or completely, a phenomenon known as the vanishing twin. Some doctors believe that because of unobserved vanishing twins, miscarriages, and induced abortions, the number of twin conceptions has been underestimated.

Doctors carefully monitor the fetal development of multiple fetuses to ensure their health and, especially, to prepare for delivering them. Amniocentesis and genetic tests detect potential biochemical defects, genetic anomalies, and diseases. Ultrasonography allows doctors to identify defects in shape and the relative position of the fetuses in the womb. Their position is important during labor. Normally, babies are born head-first. In multiples, one of the fetuses frequently lies crosswise, feet-first (complete breech), or buttocks-first (frank breech). These positions greatly lengthen and complicate delivery, making it difficult to move the baby's head through the birth canal and greatly increasing the risk of cord prolapse, which slows the delivery of oxygen and blood to the baby during labor. Moreover, the mother's over distended uterus, unable to contract properly after delivery, might begin to bleed. If labor lasts too long it could result in dangerous maternal exhaustion. Because of such problems, many obstetricians recommend delivery by cesarean section when babies are in breech position shortly before the onset of or during labor.

The prematurity and low birth weight common in multiple infants means that they are more likely to be placed on life support. Studies have found that multiple infants suffer congenital defects as much as three times more often than singletons. Identical siblings are the most likely of all to have abnormalities. Heart malformations are most common. According to some studies, closed esophagus, clubfeet, excess fingers or toes, and Down syndrome and other developmental delays occur at a slightly higher rate.

Conjoined twins are relatively rare, occuring in 1 in 50,000 or 1 in 100,000 births. Most are attached at the back, or at the back of the head or neck. In an extreme rarity, one identical twin has a full set of chromosomes while the other has only the X chromosome from the mother; in this case the twin will be a female with a condition called Turner syndrome. Therefore, identical twins will be of opposite gender if the first twin has the XY chromosomes that define a male and the other is a female with Turner syndrome.

That multiple birth siblings develop in the same environment and from a common origin allows researchers to trace the genetic and environmental influences on human development in general through twin studies. Most of this research has been conducted on twins because of the relative rarity of triplets or larger groups of siblings. The reasoning is straightforward. In the case of identical twins, either their genetic heritage (nature) or the environment (nurture) dominates in determining how they grow mentally and physically. Researchers have tested the idea by tracking down identical twins that were separated while young, usually at birth, and reared separately. If genetics control development, then the separated twins should still look and behave similarly. If environment predominates, then separated twins should show variations in appearance and temperament.

The reported research results have been mixed. Some separated twins do not look or act any more alike than siblings born separately. Others show an uncanny degree of similarity throughout their lives—dressing the same way, marrying at the same age, having the same number of children, and dying nearly at the same time of the same disease.

Perspective and Prospects

Worldwide, superstitions and a strong moral overtone have traditionally accompanied multiple births. Some societies viewed one twin as automatically good and the other evil and treat them accordingly as they mature. Others believed twins were shameful, a sign of corruption or promiscuity in the mother. These babies might be killed at birth or separated because of it. On the other hand, some nations believed that twins had divine origin or power over the elements or special talents for prophecy and telepathy.

Multiple siblings enjoy a special advantage: They are rarely lonely. Some twins even share their own private language, a phenomenon known as idioglossa. Their common development means increased requirements of time and money for their families. It also provides continuous opportunities for sharing and relying on each other. The many national and international organizations created by and for twins and other multiple birth siblings reflect their pride in their status.

Triplets have historically had a reasonable chance for survival. Before the advent of support equipment for premature babies, lung immaturity was the factor that usually determined survival. Surfactant is a chemical that is secreted in the seventh month of pregnancy. Without surfactant, lung tissues stick together and infants cannot breathe. Because of the combined size of all fetuses, many multiple infants are born before the seventh month of gestation. The Dionne quintuplets, who were born in 1934, were unusual in that for the first time in history, all five survived into adulthood. Modern support technology has helped several sets of sextuplets (six infants) to survive. In 1997, the same technology permitted all the McCaughey septuplets (seven infants) to survive. This was the first time such an event had occurred. (The chances of naturally conceiving sextuplets are one in eight to ten million conceptions.) In 2009, Nadya Suleman gave birth to octuplets (eight infants), who became the first octuplets to survive infancy.

Advances in the field of assisted reproduction have increased the odds of multiple pregnancies. Women who have difficulty conceiving are initially treated with drugs that cause more than one egg to be released during ovulation. This increases the chance of pregnancy but also increases the chance of carrying multiple fetuses. Couples who seek medical assistance to achieve pregnancy routinely use fertility drugs. A woman will have several fertilized eggs implanted to improve the odds of successfully initiating a pregnancy. A result of this approach, called in vitro fertilization, is an increased number of multiple births.

Twin births bear witness to the successes of modern health care. In the United States, incidents of multiple births, both fraternal and identical, have increased since the 1970s, and more multiples are surviving to adulthood. Fertility drugs may account for part of the increase, as does the trend among American women to delay childbearing until their thirties or forties. Nevertheless, improved prenatal care, better diet, advances in neonatal intensive care, and education about pregnancy and birth are as important.

Key terms:

chromosomes: the rod-shaped structures in the nucleus of a cell that carry genes

concordance: the condition among twins of having the same physical or psychological trait

dizygotes: fraternal twins; born from two ova separately fertilized by two sperm

embryo: the cells growing after conception until the eighth week of pregnancy

monozygotes: identical twins; born of a single ovum that divides after a single sperm fertilizes it

ovum: the egg cell released from the ovaries during ovulation

placenta: the membrane sac developed from the uterine wall that passes nutrients to the fetus through interconnected blood vessels

ultrasonography: an imaging technique that uses high-frequency sound waves to view fetuses in the womb, as well as other internal structures

zygote: a fertilized ovum before multicellular development begins

Bibliography

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