According to pregnancy medication category classified by FDA are defined as:
Category A: No risk in human studies
Category B: No risk in animal studies (insufficient studies in humans)
Category C: Risk cannot be ruled out. There are no satisfactory studies in pregnant women, but animal studies demonstrated a risk to the fetus
Category D: Evidence of risk (proven studies have demonstrated a risk to the fetus)
Category X: Contraindicated (proven studies in pregnant women have demonstrated a risk to the fetus and lead to fetal abnormalities)