The rising number of multiple pregnancies 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). Premature babies 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 pregnancy 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 pregnancies increasing?
About one-third of the increase in multiple pregnancies is due to the fact that more women over age 30 are having babies (2). Women in this age group are more likely than younger women to conceive multiples.

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 IVF, eggs are removed from the mother, fertilized in a laboratory dish and then transferred to the uterus. About 45 percent of ART pregnancies result in twins and about 7 percent in triplets or more (3). 

Doctors now monitor fertility treatments carefully so that women will have fewer, but healthier, babies. This involves limiting the number of embryos transferred during IVF.

In 2004, the American Society for Reproductive Medicine and the Society for Assisted Reproductive Technology issued guidelines on the best number of embryos to transfer, depending on a woman's age and other factors (4). For example, the guidelines recommend that doctors transfer no more than two embryos for women under age 35, and consider transferring only one embryo for women in this age group who are considered most likely to become pregnant.

Doctors monitor women taking certain fertility drugs with ultrasound. If ultrasound shows that a large number of eggs could be released during that treatment cycle, the doctor will stop the treatment. In fact, the rate of higher-order multiple births has remained stable since 1999 (1).

A woman 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 or tall (2, 5). African-American women are more likely to have twins than Caucasian women, and Asian women are the least likely to have twins (5).

What is the difference between identical and fraternal twins?
Identical twins (also called monozygotic twins) occur when one fertilized egg splits and develops into two (or occasionally more) fetuses. The fetuses usually share one placenta. Identical twins have the same chromosomes, so they generally look alike and are the same sex.

Fraternal twins (also called dizygotic twins) develop when two separate eggs are fertilized by two different sperm. Each twin usually has its own placenta. Fraternal twins (like other siblings) share about 50 percent of their chromosomes, so they can be different sexes. They generally do not look any more alike than brothers or sisters born from different pregnancies. Fraternal twins are more common than identical twins.

Triplets and other higher-order multiples can result from three or more eggs being fertilized, one egg splitting twice (or more) or a combination of both. A set of higher-order multiples may contain all fraternal siblings or a combination of identical and fraternal siblings.
 
How are multiple pregnancies 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 most multiples by the beginning of the second trimester.

(Sometimes a twin pregnancy that is identified very early is later found to have only one fetus. This is called “vanishing twin syndrome,” and its cause is not well understood. The surviving twin generally is not harmed.)

Other factors can alert a health care provider that a woman may be expecting twins or more. These include:

• Abnormal results on a blood test done around 16 weeks of pregnancy to screen for certain birth defects
• More than one heartbeat heard by a provider using a hand-held ultrasound device (Doppler)
• Rapid weight gain during the first trimester
• Larger uterus than expected
• Severe pregnancy-related nausea and vomiting (morning sickness)
• More fetal movement than experienced by the woman in a previous singleton pregnancy

When a health care provider suspects a multiple pregnancy, he will likely recommend an ultrasound examination to find out for sure. 

What complications occur more frequently in a multiple pregnancy?
Women who are expecting more than one baby are at increased risk of a number of pregnancy complications. The more babies a woman is carrying at once, the greater her risk. Common complications include:  

• Premature birth. More than 50 percent of twins, more than 90 percent of triplets, and virtually all quadruplets and higher multiples are born preterm (6). The length of pregnancy decreases with each additional baby. On average, most singleton pregnancies last 39 weeks; for twins, 35 weeks; for triplets, 33 weeks; and for quadruplets, 29 weeks (5).

• Low birthweight (LBW). About half of twins and almost all higher order multiples are born with low birthweight (less than 5 1/2 pounds or 2,500 grams) (3). LBW can result from preterm birth and/or poor fetal growth. Both are common in multiple pregnancies. Low-birthweight babies, especially those born before about 32 weeks gestation and/or weighing less than 3 1/3 pounds (1,500 grams), are at increased risk of health problems in the newborn period as well as lasting disabilities, such as mental retardation, cerebral palsy and vision and hearing loss. While advances in health care have brightened the outlook for these tiny babies, chances remain slim that all infants in a set of sextuplets or more will survive and thrive.

• Twin-twin transfusion syndrome. About 20 percent of identical twins who share a placenta develop this complication (7). It occurs when a connection between the two babies' blood vessels in the placenta results 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 up to 64 percent of affected babies (7). Removing the excess fluid appears to improve blood flow in the placenta and reduces the risk of preterm labor. Studies also suggest that using laser surgery to seal off the connection between the blood vessels may save a similar number of babies (7). An advantage of laser surgery is that only one treatment is needed, while amniocentesis generally must be repeated more than once.

• Preeclampsia. Women expecting twins are more than twice as likely as women with a singleton pregnancy to develop this complication, characterized by high blood pressure and protein in the urine (8). Severe cases can be dangerous for mother and baby. In some cases, the baby must be delivered early to prevent serious complications.

• Gestational diabetes. Women carrying multiples are at increased risk of this pregnancy-related form of diabetes (high blood sugar) (8). This condition can cause the baby to grow especially large, increasing the risk of injuries during vaginal delivery. Babies born to women with gestational diabetes also may have breathing and other problems during the newborn period.

Early diagnosis and management of these complications can help protect mother and babies.

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 pregnancy. Health care providers may 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).

If 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 believes 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 multiples has no signs of preterm labor, her provider may recommend cutting back on activities sometime between the 20th and 24th 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. The provider also may recommend regular ultrasound examinations starting around 20 weeks of pregnancy to check that all babies are growing at about the same rate. 

During the third trimester, the provider may recommend tests of fetal well-being. These include the non-stress test, which measures fetal heart rate when the baby is moving, and the biophysical profile, which combines the non-stress test with a special ultrasound examination. 

Should a woman expecting multiples gain extra weight?
Eating right and gaining the recommended amount of weight reduces the risk of having a premature or 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 should gain 25 to 35 pounds over nine months. Women pregnant with multiples should discuss their weight-gain goals with their health care provider. Women of normal weight who are expecting twins usually should gain 35 to 45 pounds (9). This breaks down to about 1 pound per week in the first half of pregnancy, and a little more than a pound a week for the remainder of pregnancy. Women pregnant with triplets or more may need to gain more.

The American College of Obstetricians and Gynecologists recommends that women with multiple pregnancies consume about 500 more calories a day than usual (a total of about 2,700 calories a day) (9). Women pregnant with multiples should discuss with their health care providers the number of extra calories they should eat.

Women who are carrying more than one baby 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.

Can a woman expecting multiples deliver vaginally?
The chance of a cesarean delivery is higher in twin than in singleton births. However, a pregnant woman has a good chance of having a normal vaginal delivery if both babies are in a head-down position and there are no other complications. 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. One grantee is studying the causes of conjoined (“Siamese”) twinning, with the ultimate goal of learning how to prevent this severe complication of twinning.

For more information
The Fetal Hope Foundation provides support and information about twin-to-twin transfusion syndrome. 

References

1. Martin, J.A., et al. Births: Final Data for 2003. National Vital Statistics Reports, volume 54, number 2, September 8, 2005.
2. Reddy, U.M., et al. Relationship of Maternal Body Mass Index and Height to Twinning. Obstetrics and Gynecology, volume 105, number 3, March 2005, pages 593-597.
3. Wright, V.C., et al. Assisted Reproductive Technology Surveillance–2003. Morbidity and Mortality Weekly Report, volume 55 (SS04), May 26, 2006.
4. Practice Committee of the Society for Assisted Reproductive Technology and the American Society for Reproductive Medicine. Fertility and Sterility, volume 82, number 3, September 2004, pages 773-774.
5. American Society for Reproductive Medicine. Multiple Pregnancy and Birth: Twins, Triplets, and Higher Order Multiples: A Guide for Patients. Birmingham AL, 2004, accessed 6/8/06.
6. Hoyert, D.L., et al. Annual Summary of Vital Statistics: 2004. Pediatrics, volume 117, number 1, January 2006, pages 168-183.
7. Fox, C., et al. Contemporary Treatments for Twin-Twin Transfusion Syndrome. Obstetrics and Gynecology, 2005, volume 105, number 6, pages 1469-1477.
8. American College of Obstetricians and Gynecologists (ACOG). Multiple Gestation: Complicated Twin, Triplet, and Higher-Order Multifetal Pregnancy. ACOG Practice Bulletin, number 56, October 2004.
9. American College of Obstetricians and Gynecologists (ACOG). Your Pregnancy and Birth, 4th Edition. ACOG, Washington, DC, 2004.

October 2006