We interviewed 140 pregnant women of any gestational age attending antenatal clinics at the Women's and Children's Hospital in Adelaide during September-October 1999 to elicit information about their patterns of medication use during and in the 3 months prior to their current pregnancy. Demographic information, information on women's prescribed, non-prescribed, and non-medicinal drug use during and in the 3 months prior to pregnancy, and information about both their general sources of information on medication use and their specific reasons for medication uptake/cessation during pregnancy were obtained. The women used an average of 0.7 0.8 prescribed and 2.3-2.6 non-prescribed medications (total 3.1-3.3) in the 3 pregnancy trimesters, compared with 1.0 prescribed and 2.2 non-prescribed prior to pregnancy. Use of a prescribed or non-prescribed medication was 96-97% across trimesters. Simple analgesics, vitamin/mineral supplements, and antacids were the most commonly taken medications. Antibiotics were the most commonly prescribed medication. Use of class A medications increased during pregnancy while use of non-class A medications decreased. Peri-conceptional folate supplementation was 31%. Alcohol consumption and cigarette smoking decreased after diagnosis of pregnancy. Both prescribed and non-prescribed medication use is common during all trimesters of pregnancy. However, overall use changes little compared with pre-pregnancy values. Rates of peri-conceptional folate supplementation are low.