Amniocentesis, which involves extracting amniotic fluid in order to analyze fetal cells for genetic abnormalities, is usually done during week 15 or 16 of pregnancy. Increasingly, however, in response to women who would like an earlier diagnosis, doctors are performing the procedure at 11 or 12 weeks. But getting an early answer is not without risks. Research has shown that the rate of miscarriage increases when the test is done earlier (0.5 percent at 15 or 16 weeks vs. 2.2 percent when performed before that). Now, a new study reveals that when amniocentesis is performed at 11 or 12 weeks the possibility of foot deformities in babies increases as well.

Researchers at the University of British Columbia in Vancouver compared the incidence of clubfoot among babies whose mothers had amniocentesis at 11 to 12 weeks and those who had the test at 15 to 16 weeks. Those who had the test earlier had a 1.3 percent chance that the baby would be delivered with clubfoot, compared with less than a tenth that number when amnio was performed at the standard time. The researchers speculate that the loss of amniotic fluid during the test and subsequent leakage may decrease the size of the amniotic sac, thereby squeezing the fetus's feet while they're still developing.

Given these risks, women who need an early clue about their baby's health should consider chorionic villus sampling (CVS), in which cells are extracted from the placenta and analyzed for genetic defects. CVS is usually done at 9 to 15 weeks and has a lower risk of miscarriage (about 1 percent), but also carries a slight chance of limb deformities.

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