Certain drugs contain substances called teratogens, which can cause birth defects. Teratogenic drugs can cause birth defects in three ways: 1) some cause damage to the fetus directly; 2) others compromise the functioning of the placenta, which leads to low birth weight and underdevelopment; and, lastly, 3) some medications can induce premature labor, or violent contractions which can injure the fetus.

A fetus is most vulnerable when exposed to harmful drugs during the first trimester of the pregnancy. However, some drugs can harm the fetus when taken at any time during the pregnancy. Drugs taken 20 days or more before conception often have either no effect or induce miscarriage.

The FDA uses five pregnancy categories to classify drugs according to the risk of harm to a fetus:

• Category A - Sufficient and well-controlled studies show no evidence of risk to the fetus.
• Category B - Animal studies show no evidence of risk to the fetus; no sufficient and well-controlled studies on pregnant women.
• Category C - Studies on animals indicate a possible adverse effect on the fetus, but the drug's potential benefits may outweigh the risk. Sufficient and well-controlled studies in humans do not yet exist.
• Category D - Positive evidence of risk to human fetuses. However, the use of this drug by pregnant women may be warranted in some cases.
• Category X - Positive evidence of human fetal risk; women who are or could become pregnant should not take this drug (Clomid falls under this category)