Context: There is inconsistent evidence that maternal 25-hydroxyvitamin D (25-(OH)D) deficiency may impair fetal growth.

Objective: To examine the relationship between maternal 25-(OH)D and parathyroid hormone concentrations at <16 and 28 weeks of gestation and offspring birth size.

Design: Observational study

Setting: Hospital antenatal clinic

Participants: Women with singleton pregnancies, before 16 weeks of gestation.

Interventions: None

Main outcome measure: Knee-heel length at birth.

Results: Altogether 374/475 (79%) women completed this study.

We found no evident relationship between birth size measures and maternal 25-(OH)D or PTH at recruitment (11 weeks).

Gestation length was 0.7 weeks [95% CI -1.3, -0.1] shorter and knee-heel length 4.3 mm smaller [-7.3, -1.3] in infants of 27 mothers with low 25-(OH)D (<28 nmol/L) at 28-32 weeks vs. babies whose mothers had higher concentrations. This latter difference was reduced to -2.7 mm [-5.4, -0.1]) after adjustment for gestation length, suggesting some of the apparent growth deficit is explained by shorter gestation. There was no evidence that other birth measures were affected.

Maternal PTH concentration at 28-32 weeks was positively related to knee-heel length, birth weight and mid-upper arm and calf circumferences. These associations were independent of 25-(OH)D concentration.

Conclusions: Low maternal 25-(OH)D in late pregnancy is associated with reduced intra-uterine long bone growth and slightly shorter gestation. The long-term consequences for linear growth and health require follow-up. The positive relationship between maternal PTH and measures of infant size may relate to increased mineral demands by larger babies, but warrants further investigation.