Multiple pregnancy, usually referred to as multiple gestation, is one in which more than one fetus develops simultaneously in the mother's womb.
The frequency of multiple births in the United States has been steadily increasing with advances in reproductive technologies. It is estimated that pregnancies resulting from assisted technologies have a 25–30 percent incidence of twins and a 5 percent incidence of triplets. The frequency of naturally occurring twins is approximately one in 80 births; however the frequency of multiple births in the United States for 2002 was as follows:
- twins, one in 32
- triplets, one in 583
- quadruplets, one in 9,267
- quintuplets and up, one in 58,286
Twin birth is by far the most common multiple birth. There are two types of twin pregnancy: fraternal and identical. Fraternal twins develop from two separate ova released at the same time and fertilized by two separate sperm. Fraternal twins are referred to as dizygotic twins, meaning that two unions of two gametes or male/female sex cells occurred to produce two separate embryos. Characteristically, with fraternal twins, each has its own placenta and amniotic sac. They may be the same or different sex, occur twice as frequently as identical twins, and have a mortality rate of 11.5 percent. Identical twins represent the splitting of a single fertilized zygote (union of two gametes or male/female sex cells to produce a developing embryo) into two separate individuals. Identical twins will have the same DNA, genetic material (genotype), but it may be expressed differently (phenotype). There are three ways identical twins can exist in the uterus: dichorionic-diamniotic twins; monochorionic-diamniotic twins; monochorionic-monoamniotic twins. In the instance of dichorionic-diamniotic twins, division of the fertilized egg occurs within 72 hours past fertilization, before the inner cell mass has developed. About 30 percent of identical twins have this classification, and each twin has its own chorion, amnion, and placenta. The mortality rate for this type of twinning is 9 percent. With monochorionic-diamniotic twins, division occurs in the range of four to eight days after fertilization, and the inner cell mass divides in two. The placenta has one chorion and two amnions, so each twin has its own amniotic sac. Approximately 68 percent of identical twins are in this classification, and they have a mortality rate of 25 percent. Thirdly, monochorionic-monoamniotic twins are contained in the same amniotic sac. The division of the fertilized egg in this case occurs nine to 13 days past fertilization or near the time of implantation in the uterus. Since they share an amniotic sac, they have an increased risk of their umbilical cords becoming entangled or knotted. Only 2 percent of identical twins are in this classification, and they have a mortality rate of greater than 50 percent. If a complete separation does not take place during the division process, the result is Siamese (or conjoined) twins.
The human female typically releases only one egg every menstrual cycle. A hormone called progesterone, released by the first egg to be produced, prevents any other egg from maturing during that cycle. When this control fails, fertilization of more than one egg is possible. Fertility drugs inhibit these controls, allowing multiple gestation to occur. It seems as if, however, that more pregnancies start out naturally with twins than was originally believed. The development of improved technology, such as ultrasound, has made it possible to determine more accurately the early pregnancy loss rate of twins to include both complete pregnancy loss and spontaneous resorption of one twin, frequently referred to as the vanishing twin phenomenon. Recent research suggests that 75 percent of twin pregnancies are lost before the end of the first trimester. Moreover, only about 50 percent of pregnancies diagnosed in the first trimester with twins result in the birth of two live infants.
An old adage related to multiple gestation is the human female was not meant to have more than twins because she only had two breasts for feeding. Of course, pregnancies with more than two babies have occurred throughout history. However, once the number of babies reaches three, overexpansion of a woman's uterus begins to cause difficulties. The implantation of several embryos and placentas in the endometrium of the uterus results in a competition for space and inevitably some implant in an area without good circulation. During a pregnancy, it is essential that the uterus be well perfused to sustain the fetus with nutrients and oxygen. A lack of oxygen can cause central nervous system damage in the fetuses that implanted in a less than desirable area. Since the human female was not made to carry an indefinite number of fetuses, multiple gestations can have many of the following complications:
- increased rate of spontaneous abortion
- two to three times greater risk of developing severe hypertension or preeclampsia (increased blood pressure)
- maternal anemia due to increased fetal demands
- premature rupture of membranes (bag of water)
- incompetent cervix (cervix opens due to pressure)
- intrauterine growth restriction of one or more fetuses
- preterm labor due to overstretched uterus
- abnormal fetal presentations
- need for cesarean section
- rare complications with twins, such as twin-to-twin-transfusion syndrome (one fetus receives more nutrients than the other due to more blood vessels perfusing one baby)
- conjoined twins
- postpartum hemorrhage
Causes and symptoms
Twinning seems to run in some families, is mainly confined to fraternal (dizygotic twins) and seems to be entirely a property of the mother, not the father. The primary cause is an increased chance of multiple ovulation, when a woman releases two or more eggs. Another major factor is maternal age; a woman who gives birth at 37 is four times more likely to have fraternal twins than at age 18. The 37-year-old is also more likely to be unable to conceive, since many women's ovaries are already starting to fail at that age. The third major factor is race; West Africans are ten times more likely to have fraternal twins than Chinese or Japanese, with Caucasians intermediate. This increased chance is also seen in African Americans. In addition, the more pregnancies a woman has had, the greater her chances of having twins. In fact, by the fourth or fifth pregnancy, the likelihood of having twins is four times higher than it was for the first pregnancy.
The use of assisted reproduction techniques, particularly ovarian stimulation, has caused a dramatic increase in the number of twin and higher multiple births. The normal process of single ovulation is interrupted because fertility drugs permit more than one egg at a time to mature and be released. The first drug to be used for this was clomiphene (Clomid). This was followed by the development of two natural hormones, follicle-stimulating hormone and chorionic gonadotrophin (Pergonal) to produce multiple eggs ovulation. The chance of multiple gestation with in vitro fertilization (IVF) is about the same as with the use of fertility drugs, because several embryos are inserted into the womb to increase the odds of conception. Similarly, other fertility techniques such as gamete intrafallopian transfer (GIFT) and zygote intrafallopian transfer (ZIFT) are also more likely to result in multiple gestations. The use of intrauterine insemination or artificial insemination (the injection of sperm into a woman's uterus with a syringe) is the only fertility treatment that does not increase the chances of conceiving multiples—of course, the woman is usually taking fertility drugs with this procedure also.
If a multiple pregnancy occurred spontaneously, the obstetrician would suspect a problem with the dates because the uterus would grow faster than usual. The gestational age of a pregnancy is determined from the first day of the last menstrual period (LMP). In a multiple gestation, the uterine measurements would be larger than dates, which normally correspond. If multiple gestation is suspected, an ultrasound may be performed to determine the gestational age of the fetus or to check for more than one fetus. With the use of assisted reproductive technology, an ultrasound is usually performed with ten days to see if any of the embryos were successful with implantation, and a multiple gestation would be revealed at that time. Following the birth of multiples, the placenta is carefully examined to determine if they are fraternal or identical. One placenta indicates identical twins.
A multiple pregnancy almost always means increased monitoring and surveillance for complications. This often means more frequent visits to the healthcare provider, serial ultrasounds to make sure that the babies are growing satisfactorily, amniocentesis to check for lung development, and close monitoring for preterm labor.
Ultrasound examinations play an important role in the care and treatment of multiple gestations. It assists with dating the pregnancy, determining the number of fetuses, detecting fetal anomalies, following the growth and development of each fetus, and serves to monitor the length of the cervix in anticipation of preterm labor. Premature birth is the constant threat of multiple gestation, and the primary threat of prematurity is related to lung development. Premature infants lack a substance, called surfactant, that permits their lungs to expand and breathe normally. If it becomes apparent that a multiple gestation is going to have a preterm delivery, the mother will be given an injection of a steroid, beta-methasone, to help the lungs mature. It is more beneficial if the steroid can be given twice in a 48 hour period; however, if there is not time for this, surfactant has been developed that can be administered into the lungs of a premature infant to facilitate breathing. An additional problem with the premature infant involves the lack of body fat. In a normal pregnancy the fetus spends the last four to six weeks growing and gaining weight, primarily body fat. This fat helps a newborn maintain his or her body temperature. Since premature infants do not have this fat, they use energy they cannot afford to stay warm.
Although research has shown that bed rest is not effective, a woman is often placed on bed rest during a multiple pregnancy to try to prevent pre-term labor and delivery. If preterm labor is impossible to control at home, the mother may be hospitalized and medication used to attempt to control contractions and dilatation of the cervix. Multiple gestations greater than twins in number are almost always delivered via cesarean section.
There are no specific treatments to alleviate medical difficulties caused by multiple pregnancies; however, there are supportive measures that may help both mother and children recover from the birthing process. There are treatments to encourage breast milk production and to combat postpartum difficulties. Various homeopathic remedies and massage can be helpful to both mother and children during the early adjustment period after birth.
Many multiple pregnancies reach fruition without difficulties; however, many do not. Despite medical advances, if the babies are born too early, they may survive but will have sequelae that limit the quality of life. If the babies are born prematurely, immediate medical care increases the chance of survival without any complications.
Amnion —Thin, tough, innermost layer of the amniotic sac.
Amniotic membrane —The thin tissue that creates the walls of the amniotic sac.
Chorion —The outer membrane of the amniotic sac. Chorionic villi develop from its outer surface early in pregnancy. The villi establish a physical connection with the wall of the uterus and eventually develop into the placenta.
Gene —A building block of inheritance, which contains the instructions for the production of a particular protein, and is made up of a molecular sequence found on a section of DNA. Each gene is found on a precise location on a chromosome.
Gestation —The period from conception to birth, during which the developing fetus is carried in the uterus.
Ova —The plural of ovum, it is the female reproductive cell.
Ovulate —To release a mature egg for fertilization.
Placenta —The organ that provides oxygen and nutrition from the mother to the unborn baby during pregnancy. The placenta is attached to the wall of the uterus and leads to the unborn baby via the umbilical cord.
Sequela —Plural, sequalae. An abnormal condition resulting from a previous disease or disorder.
Zygote —The result of the sperm successfully fertilizing the ovum. The zygote is a single cell that contains the genetic material of both the mother and the father.
Mothers with multiple pregnancy should be especially careful to get adequate prenatal care, including any necessary vitamins or recommended tests. Because of the extra stress on the mother's body, increased rate of complications, and threat of prematurity, the mother should be vigilant in making sure she gets enough rest, reduces stress, and maintains a healthy diet.
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Association of Women's Health, Obstetric and Neonatal Nursing. 2000 L Street, N.W. Suite 740, Washington, DC 20036. Web site: http://www.awhonn.org.
American College of Obstetricians and Gynecologists. 409 12th Street, S.W., P. O. Box 96920, Washington, DC 20090.
Questions about Multiple Pregnancy. [cited March 6, 2005]. Available online at: http://www.marvelousmultiples.com/abtmultpreg.htm .
Linda K. Bennington, MSN, CNS