Objective To determine whether there is a reduction in the risk of IUGR with folic acid supplementation.

Design A retrospective cohort study using the West Midlands Perinatal Institute population based database.

Setting West Midlands, UK.

Participants Births to West Midlands residents (July 2009-June 2012). Multiple pregnancies and congenital anomalies were excluded.

Main Outcome Measures Prevalence and relative risk of IUGR, defined as birth weight <10^th customised centile with 95% confidence intervals.

Results There were n = 117260 births with data for folic acid supplementation antenatally, of which 85% of women reported taking folic acid. Nullips constituted 42.6% of the cohort overall and 44% of those that took folic acid antenatally. For those women where the dose of folic acid was recorded (n = 42537), 95% took a dose of 400 mcg, 4% at 5 mg and 1% at other dose. For timing of folic acid supplementation, 26% commenced pre-conception, 34% at <5 weeks, 35% at 5–10 weeks and 5% at a later gestation. There were n = 60077 cases with complete pregnancy and demographic data allowing a logistic regression analysis adjusted for maternal age, smoking, hypertension, deprivation, ethnicity, employment status, diabetes (including gestational), BMI, single/partner, drug use, father blood relation, time of booking and parity. The risk of IUGR for women with no folic acid supplementation was prevalence13%, RR 1.09 (1.03–1.16), p < 0.01. For women that took folic acid, only the 400 mcg dose taken pre-conception showed a significant reduction, prevalence 9.7%, RR 0.90 (0.83–0.98) p = 0.01.

Conclusion Folic acid supplementation pre-conception significantly reduces the risk of IUGR.