Trying to get pregnant
Multiples: twins, triplets and beyond
In the past 2 decades, the rate of multiple births in the United States jumped dramatically. The rate of twin births increased by 70 percent between 1980 and 2004, and the rate of higher-order multiples (triplets or more) increased four-fold between 1980 and 1998 (1).
However, the rapid rise in multiple birth rates may be ending. In 2005 and 2006, the rate of twin births remained stable (1). The rate of higher-order multiple births has declined 21 percent since its peak in 1998 (1).
Today, more than 3 percent of babies in this country are born in sets of two, three or more; about 95 percent of these multiple births are twins (1). The high 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 premature birth (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 did the multiple pregnancy rate increase?
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 treatments, including fertility-stimulating drugs and assisted reproductive technologies (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 44 percent of ART pregnancies result in twins, and about 5 percent in triplets or more (3).
Doctors now monitor fertility treatments carefully so that women have fewer, but healthier, babies. This involves limiting the number of embryos transferred during IVF. In 2006, the American Society for Reproductive Medicine and the Society for Assisted Reproductive Technology issued updated 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 a treatment cycle, the doctor can stop the treatment and counsel the woman accordingly.
Who is most likely to have multiples?
The following factors can increase the chances that a woman will conceive multiples:
- Fertility treatment
- Age over 30 years
- A personal or family history of fraternal (non-identical) twins
- Obesity or taller-than-average height (2, 5)
- African-American race (African-American women are more likely to have fraternal twins than caucasian women, and Asian women are the least likely to have fraternal 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 genes, so they generally look alike and are the same sex. A woman's chances of having identical twins are not related to age, race or family history.
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 genes, 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 a multiple birth, today most parents-to-be learn the news fairly early. A routine first-trimester ultrasound can detect most multiples.
(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:
- Rapid weight gain during the first trimester
- The uterus being larger than expected
- Severe pregnancy-related nausea and vomiting (morning sickness)
- More than one heartbeat heard by a provider using a hand-held ultrasound device (Doppler)
- More fetal movement than the woman experienced in a previous singleton pregnancy
- Abnormal results on maternal blood screening done around 16 weeks of pregnancy to screen for certain birth defects
A health care provider who suspects a multiple pregnancy most likely recommends that the woman have an ultrasound 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: About 60 percent of twins, more than 90 percent of triplets, and virtually all quadruplets and higher-order multiples are born premature (1). The length of pregnancy decreases with each additional baby. On average, most singleton pregnancies last 39 weeks; for twins, 35 weeks; for triplets, 32 weeks; and for quadruplets, 29 weeks (1).
Low birthweight (LBW): More than half of twins and almost all higher-order multiples are born with low birthweight (less than 5½ pounds or 2,500 grams) (1). LBW can result from premature birth and/or poor fetal growth. Both are common in multiple pregnancies.
LBW 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 intellectual disabilities, 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.
Twin-twin transfusion syndrome (TTTS): About 10 percent of identical twins who share a placenta develop this complication (6, 7). TTTS occurs when a connection between the two babies' blood vessels in the placenta causes one baby to get too much blood flow and the other too little. Until recently, severe cases often resulted in the loss of both babies.
TTTS now can be treated with laser surgery to seal off the connection between the babies' blood vessels. It also can be treated with serial (repeated) amniocentesis to drain off excess fluid. Removing the excess fluid appears to improve blood flow in the placenta and reduces the risk of preterm labor. Both procedures can greatly improve the outlook for the babies.
However, recent studies suggest that laser surgery may save more babies and cause fewer neurological problems (such as cerebral palsy) in survivors than amniocentesis (6, 8). For example, a European study found a 76 percent survival rate for at least one fetus after laser surgery compared to 56 percent for serial amniocentesis (8). Another 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, protein in the urine and generalized swelling (edema) (9). 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) (9). This condition can cause the baby to grow especially large, increasing the risk of injuries to mother and baby during vaginal birth. 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 pregnancy?
Women who are expecting multiples generally need to visit their health care providers more frequently than women expecting one baby. These extra visits can 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 monitors the pregnant woman carefully for signs of preterm labor. The provider may do an internal exam or recommend a vaginal ultrasound to see if the woman's cervix is shortening (a possible sign that labor may begin soon).
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 reduce her 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 regularly checks the pregnant woman's blood pressure for preeclampsia. The provider also may recommend regular ultrasounds 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
- The biophysical profile, which combines the non-stress test with an ultrasound
How is preterm labor treated?
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 most likely recommends treating the pregnant woman with drugs called corticosteroids. These drugs help speed fetal lung development and reduce the likelihood and severity of breathing and other problems in premature babies during the newborn period.
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 LBW baby in singleton, as well as multiple, gestations. A healthy weight gain is especially important if a woman is pregnant with multiples because they have a higher risk of premature birth and LBW than singletons.
Women who begin pregnancy at a normal weight and who are expecting one baby usually should gain 25 to 35 pounds over 9 months. Women of normal weight who are expecting twins usually should gain 37 to 54 pounds; overweight women, 31 to 50 pounds; and obese women, 25 to 42 pounds (10). For normal-weight women, 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. Women pregnant with multiples should discuss their weight-gain goals with their health care provider.
The American College of Obstetricians and Gynecologists (ACOG) recommends that women with multiple pregnancies consume about 500 more calories a day than usual (a total of about 2,700 calories a day) (11). Women pregnant with multiples should discuss with their health care providers the number of extra calories they should eat.
Women pregnant with multiples 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 premature birth.
Can a woman expecting multiples deliver vaginally?
The chance of a cesarean birth 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 birth usually is 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 premature birth. Although these studies generally focus on singleton pregnancies, the largely unknown mechanisms leading to preterm birth of singletons and multiples may be much the same. A recent grantee has been studying the causes of conjoined ("Siamese") twinning, with the ultimate goal of learning how to prevent this severe complication.
- Martin, J.A., et al. Births: Final Data for 2006. National Vital Statistics Reports, volume 57, number 7, January 7, 2009.
- 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.
- Wright, V.C., et al. Assisted Reproductive Technology Surveillance–2005. Morbidity and Mortality Weekly Report, volume 57 (SS05), June 20, 2008.
- Practice Committee of the Society for Assisted Reproductive Technology and the American Society for Reproductive Medicine. Fertility and Sterility, volume 86, Supplement 5, November 2006, pages S51-52.
- American Society for Reproductive Medicine. Multiple Pregnancy and Birth: Twins, Triplets, and Higher Order Multiples: A Guide for Patients. Birmingham AL, 2004, accessed 12/16/08.
- Rossi, C., and D’Addario, V. Laser Therapy and Serial Amnioreduction as Treatment for Twin-Twin Transfusion Syndrome: A Metaanalysis and Review of Literature. American Journal of Obstetrics and Gynecology, volume 198, number 2, February 2008, pages 147-152.
- Lopriore, E. et al. Risk Factors for Neurodevelopment Impairment in Twin-Twin Transfusion Syndrome Treated With Fetoscopic Laser Surgery. Obstetrics and Gynecology, volume 113, number 2, February 2009, pages 361-366.
- Senat, M.V., et al. Endoscopic Laser Surgery versus Serial Amnioreduction for Severe Twin-to-Twin Transfusion Syndrome. New England Journal of Medicine, volume 351, number 2, July 8, 2004, pages 136-144.
- American College of Obstetricians and Gynecologists (ACOG). Multiple Gestation: Complicated Twin, Triplet, and Higher-Order Multifetal Pregnancy. ACOG Practice Bulletin, number 56, October 2004.
- Institute of Medicine. Weight Gain During Pregnancy: Reexamining the Guidelines. May 2009. New York: National Academy of Sciences, accessed 12/8/09.
- American College of Obstetricians and Gynecologists (ACOG). Having Twins: Patient Education Pamphlet. ACOG, Washington, DC, 2004, accessed 12/9/08.
March 2009/December 2009