Vitamin D deficiency is common. What the optimum level of vitamin D in pregnancy and whether vitamin D supplementation in pregnancy confers improved health benefits remain controversial.

To assess vitamin D status in pregnant women in a maternity service that recommends routine antenatal screening and advises supplementation where necessary, and to assess relationships between early pregnancy vitamin D levels and changes in vitamin D across pregnancy with pregnancy outcomes.

Materials and Methods
Vitamin D serum concentrations were measured in early and late pregnancy. The relationships between initial vitamin D status, maternal factors and pregnancy outcomes were estimated. Change in vitamin D over pregnancy was quantified. The relationship between change in vitamin D over pregnancy and pregnancy outcomes was also estimated.

Of 1550 women, 849 (55%) were vitamin D deficient (<50 nmol/L), 571 (37%) were insufficient (50–74 nmol/L), and 130 (8%) were replete (≥75 nmol/L) in early pregnancy. Factors associated with deficiency were increased body mass index, pregnancy in either winter or spring months, and maternal country of birth (South‐East, South and East Asia, and Africa). Vitamin D deficiency or insufficiency in early pregnancy was significantly associated with developing gestation diabetes mellitus. Levels of vitamin D significantly increased over pregnancy among nonreplete women. Increasing vitamin D over pregnancy was not related to pregnancy outcomes. Conclusion Vitamin D ‘deficiency’ is common but may not be associated with most adverse pregnancy outcomes. Routine vitamin D testing of all pregnant women does not appear warranted.