To evaluate the prevalence of vitamin D deficiency in pregnant women and their newborns in Beijing, China and the influence of vitamin D deficiency on birth size.

A cross-sectional study.

Data were collected from pregnant women who delivered during April to May 2010 at 306 Hospital of PLA in Beijing, China.

Participants in the study were seventy healthy nulliparous pregnant women with singleton pregnancies who delivered healthy babies at full term and their newborns.

Severe vitamin D deficiency (25-hydroxyvitamin D (25(OH)D) < 25 nmol/l) was detected in 54·5 % of mothers and 46·6 % of newborns. Neither mothers nor newborns had serum 25(OH)D concentrations that reached the normal level (>75 nmol/l). The concentration of 25(OH)D in mothers was positively correlated with that in cord blood (r = 0·89, P < 0·001). Newborns of mothers with severe vitamin D deficiency had lower birth length and birth weight. The head circumference and birth weight were lower in vitamin D-deficient newborns.

The study indicates that pregnant women and neonates residing in Beijing are at high risk of vitamin D deficiency. Neonatal 25(OH)D concentrations are dependently related to maternal 25(OH)D levels. Maternal and neonatal vitamin D status influences newborn size.