Women have their vitamin D status assessed after ultrasound scanning in the twelfth week of pregnancy. Women with circulating 25(OH)-vitamin D levels 25-100nmol/l will be randomised in a double-blind design to either oral vitamin D supplement (1000iu/day) or placebo at 14 weeks. Questionnaire data will include parity, sunlight exposure, dietary information, cigarette and alcohol consumption. Women will be reviewed at 19 and 34 weeks when maternal anthropometry will be assessed and blood samples taken to measure 25(OH)-vitamin D, PTH and biochemistry. At delivery venous umbilical cord blood will be collected, together with umbilical cord and placental tissue. The baby will undergo DXA assessment of bone mass within the first 14 days after birth, and followed up regularly through childhood, with a repeat DXA assessment at 4 years.