Objective: Vitamin D deficiency affects 1 billion people globally. It has an important role in bone homeostasis, braindevelopment and modulation of the immune system and yet the impact of antenatal vitamin D deficiency on infantoutcomes is poorly understood. We assessed the association of 25- hydroxyvitamin D levels (25-OHD) in late pregnancy andearly infant growth and developmental outcomes in rural Vietnam.Design and Methods: A prospective cohort study of 960 women who had previously participated in a double-blind clusterrandomized controlled trial of antenatal micronutrient supplementation in rural Vietnam was undertaken. Maternal 25-OHDconcentration was measured at 32 weeks gestation, and infants were followed until 6 months of age. Main outcomemeasures were cognitive, motor, socio-emotional and language scores using the Bayley Scales of Infant Development, 3rdedition, and infant length-for-age z scores at 6 months of age.Results: 60 (582/960) of women had 25-OHD levels ,75 nmol/L at 32 weeks gestation. Infants born to women with 25-OHD deficiency (,37.5 nmol/L) had reduced developmental language scores compared to those born to women who werevitamin D replete ( 75 nmol/L) (Mean Difference (MD) 23.48, 95 Confidence Interval (CI) 25.67 to 21.28). For every25 nmol increase in 25-OHD concentration in late pregnancy, infant length-for-age z scores at 6 months of age decreasedby 0.08 (95 CI 20.15 to 20.02).Conclusions: Low maternal 25- hydroxyvitamin D levels during late pregnancy are of concern in rural Vietnam, and areassociated with reduced language developmental outcomes at 6 months of age. Our findings strengthen the evidence forgiving vitamin D supplementation during pregnancy.