Low maternal vitamin D status has been associated with reduced intrauterine long bone growth and shorter gestation, decreased birth weight, as well as reduced childhood bone-mineral accrual. Despite data from other countries indicating low maternal vitamin D status is common during pregnancy, there is a dearth of information about vitamin D status during pregnancy in the Irish female population. Therefore, we prospectively assessed vitamin D nutritive status and the prevalence of suboptimal vitamin D status in a cohort of Irish pregnant women. The mean (SD) daily intake of vitamin D by the group of pregnant women was 3.6 (1.9) microg/day. None of the women achieved the recommended daily vitamin D intake value for Irish pregnant women (10 microg/day). Taking all three trimesters collectively, 14.3-23.7% and 34.3-52.6% of Irish women had vitamin D deficiency (serum 25 (OH) D <25 nmol/l) and insufficiency (serum 25 (OH) D 25-50 nmol/l), respectively during pregnancy. Both the levels of serum 25 (OH) D and the prevalence of vitamin D deficiency/adequacy were dramatically influenced by season, with status being lowest during the extended winter period and best during the extended summer period. These findings show that inadequate vitamin D status is common in Irish pregnant women.