When are birth defects most likely to occur during pregnancy?

Birth defects are most likely to occur during the first three months of pregnancy, specifically during the first trimester. This period is crucial as it encompasses the early stages of fetal development, during which major organs and structures are forming. The embryo is highly vulnerable to teratogens—substances that can cause birth defects—during this time. Environmental factors, maternal health, and genetic factors can significantly impact the developing fetus, making the first trimester a critical window for the occurrence of structural and functional defects.

As development progresses into the second and third trimesters, while there can still be risks for certain conditions, the major formation of physical structures has largely completed. Most birth defects that are detected later are either related to genetic factors or complications arising during later stages rather than the result of teratogenic effects that are most impactful during the early weeks of pregnancy. Thus, the most critical time frame for the occurrence of birth defects is indeed during the first three months.