Which substance poses the highest risk for birth defects if used during pregnancy?

Alcohol poses the highest risk for birth defects if used during pregnancy because it is associated with a range of serious outcomes known as fetal alcohol spectrum disorders (FASDs). These disorders can result in physical, behavioral, and cognitive issues in the child, including facial deformities, growth deficiencies, and neurological impairments. Alcohol can cross the placenta and affect the developing fetus' brain and other organs, leading to lifelong consequences.

While substances like cocaine, marijuana, and tobacco can also have harmful effects on fetal development and contribute to various complications, the evidence surrounding alcohol and its direct relationship with a range of developmental disorders is more pronounced. The teratogenic effects of alcohol are well-documented, making it particularly concerning during pregnancy compared to the other substances listed.