When does pregnant women become more vulnerable to teratogens?

Pregnant women are most vulnerable to teratogens during the first trimester, particularly between weeks 3 and 8 of gestation, when the foundational structures of the fetus are forming. During this critical period, exposure to harmful substances like medications, alcohol, or infections can lead to significant developmental malformations. However, the vulnerability to teratogens can also persist into the second and third trimesters, particularly as the brain and other organs continue to develop. Overall, the timing and type of teratogen exposure play crucial roles in potential effects on fetal development.