OBJECTIVES: The US Food and Drug Administration (FDA) has classified drugs used in pregnancy into five categories namely A, B, C, D and X with category A being the safest and category X being absolutely contraindicated in pregnancy based on fetal risk associated with these medications. The objective of the study was to determine the time trend and predictors of FDA’s category D and X drugs among outpatient visits in pregnant women. METHODS: A retrospective study using the 1997-98 and 2005-06 National Ambulatory Medical Care Survey and the outpatient department component of the National Hospital Ambulatory Medical Care Survey was conducted to examine maternal characteristics and physician characteristics associated with Category D and X prescriptions. Descriptive statistics and logistic regression analysis were performed to determine the time trend and predictors associated with prescribing category D and X drugs. RESULTS: Approximately 35 million visits were made by pregnant women annually during these years. Of these visits, 1.4% (95%Confidence Interval (CI):1.13-1.77%) of the visits involved prescription of Category D drugs and 1.1% (CI:0.6-1.4%) of the visits involved prescription of Category X drugs.  A significant positive time trend was observed in prescribing of category D and X drugs in 2005-06 (OR-2.14, 95%CI:1.23-3.73) compared to 1997-98. Pregnant women above the age of 30 had greater odds (OR-1.62, 95%CI:1.08-2.44) of receiving these drugs than pregnant women below the age of 30. Pregnant women who received ≥2 medications had greater odds (OR-15.31,95%CI:9.02-26.01) of receiving these drugs than who received ≤1 medication. Significant differences across physician specialty and race were also observed. CONCLUSIONS: Both maternal and physician characteristics play a role in prescribing of Category D and X drugs. With increase in use of category D and X drugs, more research is needed to evaluate the clinical consequences of these prescribing practices.