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Multiplying Multiples

Multiple births seem to dominate the news and celebrity magazines. That's not surprising. During the past 25 years or so, the number of twin births in the United States has increased 65 percent, and the number of higher-order multiples (triplets or more) has increased by more than 400 percent, according to the National Center for Health Statistics. Approximately three of every 100 babies born in the United States are multiples.

Why the increase? Part of the reason is that women in the United States are waiting longer to have babies. As a woman ages, her chance of having multiples goes up. Assisted reproduction techniques (ART) such as in vitro fertilization also contribute to the rise in multiples. About half of the births that result from ART are multiples.

Your likelihood of multiples. Multiples are more likely to be born to mothers who have them in their family. According to recent studies, maternal height and weight come into play too. Women in the top 25th percentile of height and those with a body mass index of 30 or greater (which is considered obese) are more likely than thinner, shorter women to have multiples.

Twins are most common. The vast majority of multiple births are twins; they account for about 95 percent of multiple births. Twins can be identical or fraternal. Identical twins, which account for one-third of twin sets, occur when one fertilized egg splits early in pregnancy and develops into two fetuses. With fraternal twins, each fetus develops from a separate egg and sperm. Identical twins share one placenta and amniotic sac; fraternal twins each have their own placenta and amniotic sac.

The chance of having identical twins versus fraternal twins is not linked to age, family history, or anything else that scientists have uncovered so far. Having identical twins is believed to be an unpredictable, random event.

Detecting multiples. In your mother's day, a woman discovered she was having multiples in the delivery room, when the doctor pulled out one baby and realized that the party wasn't over yet. Now ultrasounds show multiples as early as 6 weeks into pregnancy and can detect more than 95 percent of multiples by the beginning of the 2nd trimester. If you don't have an ultrasound, or if you have one later in your pregnancy, your doctor may suspect multiples after hearing more than one heartbeat. Multiples are also a possibility if you are gaining a lot of weight, if your uterus is much larger than your gestational age would suggest, or if you have severe morning sickness. Multiples may be in your future if you have abnormal results on the multiple marker screening test, also known as the quad marker screen; this blood test is done between weeks 15 and 20.

Risks and precautions. Women who are pregnant with multiples see their doctors more often. Their pregnancies are usually considered high-risk because they have a greater chance of miscarrying one or more of the fetuses. They're also at higher risk for premature labor, gestational diabetes, preeclampsia, iron deficiency anemia, and intrauterine growth restriction, which indicates that a fetus is growing poorly. Multiples usually arrive early: 60 percent of twins, 90 percent of triplets, and virtually all multiples greater than triplets are born preterm. Here's a quick rule: On average, singletons arrive at 40 weeks, twins at 36 weeks, triplets at 32 weeks, and quadruplets at 28 weeks.

Multiples are often born by cesarean delivery because premature babies are less likely to be vertex (in the head-down position). Almost all high-order multiples (three babies or more) are delivered by cesarean. Many twins may be delivered vaginally, especially if baby A (the baby that is presenting) is head down and baby B is smaller or the same size as baby A.

Originally published in You & Your Baby: Pregnancy.

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