During which period of pregnancy are babies most vulnerable to drugs taken by the mother?

A) First trimester
B) Second trimester
C) Third trimester
D) None of these; vulnerability stays the same throughout gestation.

The first trimester, which is roughly the first 12 weeks of pregnancy, is the period when the baby’s organs are forming and developing. During this period, drugs and other substances taken by the mother can cross the placenta and harm the baby's growth and development, leading to birth defects and other health problems.