For example, measurement of acetylcholinesterase and amniotic fluid alpha-fetoprotein can also be performed on amniotic fluid. These two tests assist in prenatally diagnosing spina bifida and some other birth defects. This procedure is usually performed at 15-17 weeks gestation. Results generally return in 7-14 days. There are risks to the procedure.
Fetal loss is the worst risk.
In general, amniocentesis is a safe procedure that yields accurate information. There are advantages to knowing the fetus' genetic test results earlier in pregnancy than 16-18 weeks gestation. However, there are also increased risks when performing the procedure before 14 weeks. Another way of obtaining fetal tissue is to take a small amount of the placental tissue from within the uterus. Placental tissue should have the same genetic composition as the fetus.