Final answer:
The best time for an amniocentesis to check for Tay-Sachs disease is at 14 to 16 weeks' gestation when it is safest and most reliable.
Step-by-step explanation:
The optimum time for a pregnant woman to have an amniocentesis to determine if the fetus has Tay-Sachs disease is B. 14 to 16 weeks' gestation. This procedure involves extracting a small amount of amniotic fluid which contains fetal cells to examine the fetal chromosomes for any genetic abnormalities. It is typically performed in the second trimester when sufficient amniotic fluid is available and the risk to the fetus is lower compared to earlier in pregnancy.