Third Trimester: Amniocentesis The test is most often offered
between 15 and 20 weeks of gestation (usually around week 16) to
a pregnant woman under any of the following circumstances: - She
is 35 years of age or older. - There is a strong family history
of a genetic disorder from the parents. - Both parents possess a
gene for an inherited disorder. - The level of maternal alpha-
fetoprotein (AFP) is either lower or higher than expected. - A
previous child in the family had a birth defect or chromosome
problem. Amniocentesis may also be performed after 32 weeks of
gestation to help assess the degree of lung development for
babies at risk of premature delivery. During the procedure, a
needle is inserted through the walls of the abdomen and uterus
and into the thin-walled sac of fluid that surrounds the
developing fetus. A small amount of amniotic fluid is withdrawn.
This fluid contains AFP produced by the baby and fetal cells.
The fetal cells can be tested for chromosomal or genetic
abnormalities. A gene analy