Pregnant women and their partners can already peer at an unborn child's chromosomes: with amniocentesis, they can learn about the presence or, more likely, absence of large-scale genetic defects, often gaining peace of mind. But only a small percentage of parents-to-be take the opportunity, because the procedure is invasive and uncomfortable-a large needle is inserted into the amniotic sac-and causes miscarriage in roughly one in 400 cases.
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