A multiple gestation is a pregnancy in which a woman is carrying two or more babies (fetuses). In the past two decades, the number of multiple births in the United States has jumped dramatically. Between 1980 and 2000, the number of twin births has increased 74 percent, and the number of higher order multiples (triplets or more) has increased fivefold, according to the National Center for Health Statistics. Today, about 3 percent of babies in this country are born in sets of two, three or more, and about 95 percent of these multiple births are twins.
The rising number of multiple gestations is a concern because women who are expecting more than one baby are at increased risk of certain pregnancy complications, including preterm delivery (before 37 completed weeks of pregnancy). Babies who are born preterm are at risk of serious health problems during the newborn period, as well as lasting disabilities and death.
Some of the complications associated with multiple gestation can be minimized or prevented when they are diagnosed early. There are a number of steps a pregnant woman and her health care provider can take to help improve the chances that her babies will be born healthy.
Why are multiple gestations increasing?
About one-third of the increase in multiple gestations is due to the fact that more women over age 30 (who are more likely to conceive multiples) are having babies. The remainder of the increase is due to the use of fertility-stimulating drugs and assisted reproductive techniques (ART) such as in vitro fertilization (IVF)(in which eggs are removed from the mother, fertilized in a laboratory dish and then transferred to the uterus). According to the most recent survey of ART programs in the United States, 56 percent of births resulting from these procedures were multiples.
Doctors now realize that it is crucial to monitor fertility treatments so that women will have fewer, but healthier, babies. This involves limiting the number of embryos transferred during IVF, or halting treatment with fertility drugs during a cycle if ultrasound shows that a large number of eggs could be released. In fact, the rate of higher-order multiple births has declined slightly in the past two years.
A woman also has a higher-than-average chance of conceiving twins if she has a personal or family history of fraternal (non-identical) twins or if she is obese.
How are multiple gestations diagnosed?
Although previous generations often were surprised by the delivery of twins (or other multiples), today most parents to-be learn the news fairly early. An ultrasound examination can detect more than 95 percent of multiples by the beginning of the second trimester. (Sometimes a seemingly normal twin gestation that is identified very early is later found to have only one fetus. These so-called “vanishing twin” events are not well understood.)
An abnormal result on the triple or quadruple screen—blood tests done around the 16th week of pregnancy to identify babies at increased risk of certain birth defects—also alerts a health care provider to the possibility of multiples, as does hearing more than one fetal heartbeat during a routine examination. A provider also may suspect that a woman is carrying more than one baby if she puts on weight more rapidly than anticipated in the first trimester, her uterus is larger than expected or if she has severe pregnancy-related nausea and vomiting (morning sickness). Some women also may notice more fetal movement than in a previous singleton pregnancy. Whenever a multiple gestation is suspected, the health care provider will most likely recommend an ultrasound examination to find out for sure.
What complications occur more frequently in a multiple gestation?
Twins generally face the fewest medical complications and are usually born healthy. The more babies a woman carries at once, the greater her risk of complications.
Close to 60 percent of twins, over 90 percent of triplets and virtually all quadruplets and higher multiples are born preterm. The length of gestation decreases with each additional baby. On average, most singleton pregnancies last 39 weeks; for twins, 36 weeks; for triplets, 32 weeks; for quadruplets, 30 weeks; and for quintuplets, 29 weeks.
Most preterm multiples weigh less than 5 1/2 pounds (2,500 grams), which is considered low birthweight. Low-birthweight babies, especially those born before 32 weeks of gestation and/or weighing less than 31/3 pounds (1,500 grams), are at increased risk of health complications in the newborn period as well as lasting disabilities, such as mental retardation, cerebral palsy and vision and hearing loss. While advances in caring for very small infants has brightened the outlook for these tiny babies, chances remain slim that all infants in a set of sextuplets or more will survive and thrive.
Before birth, identical twins face an additional risk. One-third of all twin pairs are identical—they begin as one fertilized egg that subsequently divides in half. The remaining two-thirds of twins are fraternal, resulting from two different eggs fertilized by two different sperm. Fraternal twins are no more similar genetically than are any other siblings. They may not be the same sex; they may not even look alike. Higher order multiples can result from three (or more) eggs being fertilized, one egg splitting twice (or more) or a combination of both.
Identical twin fetuses have a 15 percent chance of developing a serious complication called twin-to-twin transfusion syndrome. This condition, which occurs when there is a connection between the two babies’ blood vessels in their shared placenta, can result in one baby getting too much blood flow and the other too little. Until recently, severe cases often resulted in the loss of both babies. Recent studies, though, suggest that the use of amniocentesis to drain off excess fluid can save about 60 percent of affected babies. Removing the excess fluid appears to improve blood flow in the placenta and reduces the risk of preterm labor. Recent studies also suggest that using laser surgery to seal off the connection between the blood vessels may save a similar number of babies. An advantage of laser surgery is that only one treatment is needed, while amniocentesis generally must be repeated more than once.
Women who are pregnant with multiples also face an increased risk of pregnancy-related forms of high blood pressure (preeclampsia) and diabetes. More than half of triplet pregnancies are complicated by preeclampsia. With proper treatment, these disorders usually do not pose a major risk to mother or baby.
Should a woman expecting multiples gain extra weight?
Eating right and gaining the recommended amount of weight reduces the risk of having a low-birthweight baby in singleton, as well as multiple, gestations. A healthy weight gain is especially important if a woman is pregnant with twins or more, as multiples have a higher risk of preterm birth and low birthweight than singletons.
Women who begin pregnancy at a normal weight and who are expecting one baby usually are advised to gain 25 to 35 pounds over nine months. Women of normal weight who are expecting twins are usually advised to gain 35 to 45 pounds. Women pregnant with triplets should probably aim for a gain of 50 to 60 pounds.
Studies show that gaining enough weight in the first 20 to 24 weeks of pregnancy is especially important for women carrying multiples. In a twin pregnancy, a gain of at least 24 pounds by the 24th week of pregnancy helps reduce the risk of having preterm and low-birthweight babies. A good early weight gain may be especially important in multiple gestations because these pregnancies tend to be shorter than singleton pregnancies. Studies also suggest that a good early weight gain aids in development of the placenta, possibly improving its ability to pass along nutrients to the babies.
The American College of Obstetricians and Gynecologists recommends that women with multiple pregnancies consume about 300 more calories a day than women carrying one baby (a total of about 2,700 to 2,800 calories a day). However, women pregnant with multiples should discuss the number of extra calories they should eat with their health care providers. They also should take a prenatal vitamin that is recommended by their health care provider and that contains at least 30 milligrams of iron. Iron-deficiency anemia is common in multiple gestations, and it can increase the risk of preterm delivery.
What special care is needed in a multiple gestation?
Women who are expecting multiples generally need to visit their health care providers more frequently than women expecting one baby to help prevent, detect and treat the complications that develop more often in a multiple gestation. Health care providers usually recommend twice-monthly visits during the second trimester and weekly (or more frequent) visits during the third trimester.
Starting around the 20th week of pregnancy, a health care provider will monitor the pregnant woman carefully for signs of preterm labor. She may do an internal exam or recommend a vaginal ultrasound examination to see if the woman’s cervix is shortening (a possible sign that labor may begin soon). Some providers also do electronic uterine monitoring. Home uterine monitoring was once recommended for women at especially high risk of preterm labor, but studies of women expecting twins did not find it useful.
When a woman develops preterm labor, her provider may recommend bed rest in the hospital and, possibly, treatment with drugs that may postpone labor. If the provider does not believe labor will stop and if the babies are likely to be born before 34 weeks gestation, she will probably recommend that the pregnant woman be treated with drugs called corticosteroids. These drugs help speed fetal lung development and reduce the likelihood and severity of breathing and other problems during the newborn period.
Even if a woman pregnant with twins has no signs of preterm labor, her provider may recommend cutting back on activities sometime between the 20th and 30th weeks of pregnancy. She may be advised to cut back on activities even sooner and to rest several times a day if she is expecting more than two babies.
As a multiple gestation progresses, the health care provider will regularly check the pregnant woman’s blood pressure for preeclampsia. Regular ultrasound examinations often are recommended to check on the babies’ rates of growth— including growth differences among them, which can be signs of serious problems. During the third trimester, the provider may recommend tests of fetal well-being (such as the non-stress test, which measures fetal heart rate when the baby is moving).
Can a woman expecting multiples deliver vaginally?
While the chance of a cesarean delivery is higher in twin than in singleton births, about half of women expecting twins can have a normal vaginal delivery. Chances are good if both babies are in a normal, head-down position. However, when a woman is carrying three or more babies, a cesarean delivery is usually recommended because it is safer for the babies.
Does the March of Dimes support research relevant to multiple gestation?
The March of Dimes supports a number of grants aimed at improving understanding of the causes of preterm delivery. Although these studies generally focus on singleton pregnancies, the largely unknown mechanisms leading to preterm delivery of singletons and of multiples may be much the same.
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