The use of computerized databases to identify and follow large cohorts of pregnant women make it feasible to study trends in prescribing practices and to quickly identify defined cohorts of exposed pregnancies for further research. OBJECTIVES: This study had two purposes, to develop a methodology to identify pregnant women in an automated database and determine the type and extent of prescription drug use during a defined pregnancy period. METHODS: A retrospective analysis of prescription claims from an integrated managed care data base. The study population consisted of women, aged 12-50 years of age with a pregnancy indication during the period 10/12/96-12/31/98. Pregnant women were identified using ICD-9-CM diagnosis and procedure codes and CPT-4 codes. The pregnancy period was defined using date of service fields. RESULTS: 4,732 women were identified with a pregnancy during the specified time frame. 826 (17%) of these members had claims evidence of prescription drug acquisition. Each woman received on average 2 different medications during pregnancy and an average of 4.5 prescriptions. 25% of the prescription were agents defined as pregnancy class C (prescription anthihistmaine agents, antiinfective agents and psychiatric medications such as lithium and phenothiazines), 4% were defined as pregnancy class D (antidepressant agents such as amitriptyline and nortriptyline, anticonvulsant agents), and 7% were classified as pregnancy class X (isotretinoin and estrogen preparations). Pregnancy class X poses the largest concern. 115 women were identified with claims for oral contraceptives that did not appear during the first or last month of the during the pregnancy period pregnancy. One woman was identified with claims for isotretinoin during the pregnancy period. CONCLUSIONS: Information contained in the claims data provides evidence regarding the use of prescription drugs during pregnancy. Despite warnings in literature and public media drugs defined as pregnancy class X are acquired during the pregnancy period.