BACKGROUND: The need for omega-3 fatty acids, especially docosahexaenoic acid (DHA), during pregnancy has received much attention, but evidence of effects on birth outcomes is limited.

OBJECTIVE: To evaluate whether prenatal DHA supplementation increases gestational age and birth size.

METHODS: We conducted a double-blind, randomized, placebo-controlled trial in Cuernavaca, Mexico. We randomly assigned 1,094 pregnant women (18 to 35 years of age; median DHA dietary intake, 55 mg/day) to 400 mg/day of algal DHA or placebo from 18 to 22 weeks of gestation through delivery. Birth outcomes (968 live births and 5 stillbirths) were ascertained from hospital records within 24 hours of delivery.

RESULTS: Intention-to-treat analysis showed no differences between the control and DHA group (all p > .05) in mean gestational age (39.1 + 1.7 and 39.0 +/- 1.9 weeks, respectively), weight (3.20 + 0.47 and 3.21 +/- 0.45 kg, respectively), length (50.3 +/- 2.7 and 50.3 +/- 2.3 cm, respectively) and head circumference (34.3 +/- 1.8 and 34.3 +/- 1.5 cm, respectively) at birth. Offspring of supplemented primigravidae (n = 370) were heavier (difference, 99.4 g; 95% CI, 5.5 to 193.4) and had larger head circumferences (difference, 0.5 cm; 95% CI, 0.1 to 0.9) than controls; the differences in multigravidae (n = 603) were -53.3 g (95% CI, -126.8 to 20.2) and -0.2 cm (95% CI, -0.4 to 0.1), respectively (p < .05 for heterogeneity).

CONCLUSIONS: Prenatal DHA supplementation of primigravid women may result in increased birth size in a population where dietary DHA intakes are very low. Benefits of the intervention on infant health and neurodevelopment are under study.