In the context of prenatal care, which substance is known to cause structural abnormalities during pregnancy?

During pregnancy, exposure to certain substances can have significant effects on fetal development. Alcohol is well-documented as a teratogen, meaning it can cause structural abnormalities in the developing fetus. The consumption of alcohol during pregnancy can lead to a range of issues collectively known as Fetal Alcohol Spectrum Disorders (FASD), which are associated with physical, behavioral, and learning difficulties. Structural abnormalities may include facial dysmorphisms, congenital heart defects, and other physical malformations.

In contrast, while non-steroidal anti-inflammatory drugs like ibuprofen are discouraged during certain periods of pregnancy, particularly in the third trimester, their teratogenic effects are not as pronounced as those of alcohol. Metformin is often prescribed for gestational diabetes and is generally considered safe in pregnancy. Insulin is essential for managing diabetes in pregnancy and does not cause structural abnormalities. Thus, when focusing on substances that can cause serious and structural developmental issues in a fetus, alcohol stands out as the correct answer.