Objective To examine the prevalence of folic acid supplementation prior to conception and in the first trimester of pregnancy, and to identify sociodemographic variables associated with the use of supplements.

Design Observational study.

Setting District general hospital in the south in England.

Population Nine hundred and sixty-three randomly selected pregnant nulliparous Caucasian women recruited from May 1994 to February 1996 inclusive.

Methods Questionnaire administered at approximately 16 weeks gestation.

Main outcome measures Intakes of supplemental folic acid before conception and during pregnancy.

Results 31.5% (303/963) (95% CI 28.5–34.4) of pregnant women reported using supplements containing folic acid prior to conception. The proportion using pre-conceptional folic acid increased by approximately 1% per month during the 22 months of the study. 38.1% (367/962) (35.1 to 41.2) of women began taking folic acid only after the confirmation of pregnancy, and this proportion appeared constant over time. Young age, smoking and low educational attainment were statistically significant predictors of failure to use folic acid both before and during pregnancy.

Conclusions Use of folic acid before conception in nulliparous women is much higher than the 2% to 3% reported in earlier studies of all pregnant women, and appears to be increasing. However, many women still only begin taking folic acid after conception, despite current health education strategies. New approaches, focusing on women who are currently least likely to take folic acid - those who are young, are of low educational backgrounds, and are smokers - may now be required. Given the inevitably of unplanned pregnancies, efforts must also be made to increase the currently static uptake of folic acid immediately after the confirmation of pregnancy.