Does vitamin D during pregnancy impact offspring growth and bone?

During pregnancy, maternal and fetal Ca demands are met through increased intestinal Ca absorption. Increased Ca absorption may be more dependent on oestrogen's up-regulation of Ca transport genes than on vitamin D status. Numerous studies, however, have found that severe vitamin D deficiency with secondary hyperparathyroidism during pregnancy leads to abnormal Ca homoeostasis in the neonate. Some, but not all, studies of maternal vitamin D supplementation during pregnancy find a greater birth weight among infants of mothers with adequate vitamin D status. Observational studies find a higher incidence of small-for-gestational age (SGA) infants among mothers who are vitamin D deficient, but this effect may be modified by genetics. In addition, the effect of vitamin D status on SGA may not be linear, with increased occurrence of SGA at high maternal 25-hydroxyvitamin D (25-OHD) concentrations. Some studies, but not all, also have found that maternal vitamin D status is associated with growth trajectory during the first year of life, although the findings are contradictory. There are recent studies that suggest maternal 25-OHD, or surrogates of vitamin D status, are associated with growth and bone mass later in childhood. These results are not consistent, and blinded randomised trials of vitamin D supplementation during pregnancy with long-term follow-up are needed to determine the benefits, and possible risks, of maternal vitamin D status on offspring growth and bone development. The possibility of adverse outcomes with higher maternal 25-OHD concentrations should be considered and investigated in all ongoing and future studies.