The aim of this study was to examine serum vitamin D concentrations from early pregnancy until delivery in women who did and did not develop preeclampsia. This longitudinal study was carried out in Pune, India. A total of 1154 women with singleton pregnancies were recruited in early pregnancy from two hospitals. Blood samples were collected and stored at four time points across gestation: V1 = 11-14 weeks, V2 = 18-22 weeks, V3 = 26-28 weeks and V4 = at delivery. 108 women who developed preeclampsia (PE) and 216 who did not develop PE (Non-PE) were randomly selected from the remainder. Serum 25-hydroxy vitamin D concentrations (25(OH)D) were estimated in their samples using commercially available ELISA kits. Independent t-tests were used to compare 25(OH)D between PE and non-PE groups. Logistic and linear regressions were used to examine associations of 25(OH)D with the risk of preeclampsia and birth outcomes, respectively, after adjusting for confounders. The mean (SD) 25(OH)D at V1 was 21.95 (19.64) in the Non-PE group and 17.76 (13.21) in the PE group. A decrease in the concentrations of vitamin D (ng ml-1) in mid-pregnancy (V2) and at delivery was associated with an increased risk of preeclampsia (0.31 [95% CI 0.11, 0.86], p = 0.024 and 0.24 [95% CI 0.08, 0.77], p = 0.016), respectively. Our finding of lower vitamin D concentrations in mid-pregnancy, before women developed clinical preeclampsia, suggests that vitamin D may have a role in its pathophysiology.