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Is there a higher risk of birth defects with multiple babies?
Sharing a womb with a sibling or two doesn't affect a baby's DNA or increase the risk of a genetic defect such as Down syndrome. If you're carrying more than one baby, each one is just as likely as a single baby to be free of genetic abnormalities. But when you are carrying more than one baby, each baby has its own risk of a genetic abnormality, so as a whole your pregnancy is at higher risk.
Even if there's nothing wrong with a baby's chromosomes, twins or triplets are more likely than singleton babies to be born with defects of the brain, heart, bladder, or liver. Multiples are also more likely to be born with cerebral palsy. A recent study of more than a million births found that cerebral palsy was four times more common in twins than in singletons.
The risk is still small. A recent study found birth defects in about 35 of every 1,000 twin babies, compared to about 25 of every 1,000 singleton babies.
What prenatal screening tests can I have if I'm carrying multiples?
Prenatal screening tests can help gauge your babies' risk of chromosomal abnormalities and other birth defects. Women carrying twins can have all the same tests as women carrying singletons, although the results may be less accurate. Women carrying triplets can only have the nuchal translucency screening, a first trimester ultrasound that tells you if one or more of your babies has extra fluid in the neck, a potential sign of Down syndrome.
What about amniocentesis or CVS?
Amniocentesis and chorionic villus sampling are used to diagnose chromosomal abnormalities and other defects. These tests are often performed early in pregnancy (between 10 and 13 weeks for CVS and 16 and 22 weeks for amniocentesis) to check for Down syndrome and other genetic problems. For mothers with multiples, these tests can also determine whether babies are identical or fraternal.
Both amniocentesis and CVS are trickier if you're carrying twins, triplets, or more. Unless your babies happen to share an amniotic sac, the technician has to be very careful to collect a sample from each baby without getting the samples mixed up. (Twins who share an amniotic sac have the same DNA, so mix-ups don't matter.) If you're carrying triplets or more, it's not always possible to collect a sample from each baby.
The risk of miscarriage from these tests is also somewhat higher when you're carrying multiples. For example, nearly 3 percent of twins will miscarry after an amniocentesis, about five times the rate seen with single babies. The rate of miscarriage after CVS can be as high as 4 percent, although the risk is lower if you have an experienced technician. You'll want to thoroughly discuss the risks and benefits of these tests with your practitioner or genetic counselor before undergoing either procedure.