- Women were asked whether they had taken prenatal vitamins, multi-vitamins, or single-ingredient folic acid tablets during the periconceptual period 3 months before and after conception

- For each month during the 6 month period, interviewers recorded the type of folic acid supplement used, number of pills taken daily/weekly, and number of days/weeks in which vitamins were taken

- Prenatal vitamins were assumed to contain 1 mg of folic acid, multivitamins or single folic acid tablets were assumed to contain .4 mg

- For analysis, exposure to folic acid was defined as use of folic-acid containing vitamins during the 3 months before conception

- Reported use of vitamins during this time period is thought to closely reflect true intake during neural tube closure (~1 month postconception)

- This exposure definition is the same as the one used by Shaw

Dietary Folate

- Dietary folate intake measured with the 98-item food frequency questionnaire

- For each food item, women estimated their usual frequency of intake (average number of times per month/week/day) over the 6 month periconceptual period

- Exposure categories for dietary folate intake were based on quartiles derived from the distribution of the controls

Combined Folate Exposure