The needs of the fetus and additional maternal metabolism are believed to compromise maternal calcium homeostasis during pregnancy. Animal studies indicate that the fetal skeleton responds to changes in maternal calcium intake. Previous clinical studies have centered on populations living on restricted food supplies that are deficient in multiple nutrients. The effect of calcium deficiency or supplementation is unclear. Koo and colleagues studied the effect of calcium supplementation on the fetal skeleton during pregnancy.

More than 200 healthy women with a singleton pregnancy confirmed by ultrasound examination were recruited. The participants were randomly assigned before 22 weeks' gestation to receive 2 g of calcium or placebo daily. All women received dietary advice and were provided with a commercial antenatal vitamin in addition to the study supplements. They completed 24-hour diet diaries at the beginning of the study and again at 32 to 33 weeks' gestation. Compliance was checked by tablet counts at each visit every four weeks through gestation week 29, then every two weeks through gestation week 35. Dual-energy radiograph absorptiometry scans of the whole body and lumbar spine were performed within the first week of life on 256 infants (128 infants in each group).

Maternal body mass index, maternal age and baseline dietary calcium content were all positively associated with total fetal bone mineral content. These factors, plus compliance, were taken into account in assessing study results. The two study groups were comparable in dietary calcium intake and compliance with study medications. Overall, no significant differences were found between the two treatment groups in gestational age at delivery, birth weight or infant bone mineral content. When the data were analyzed by quintile of maternal calcium intake, however, a significant difference was seen in the total-body bone mineral content in infants born to women in the lowest quintile.

The authors calculate that in women with the lowest dietary calcium intake (approximately 600 mg per day), daily supplementation with 1,200 mg of calcium could increase fetal bone mineral content by about 15 percent. Conversely, calcium supplementation in pregnant women who have calcium intakes within the recommended range is unlikely to increase fetal bone mineral content.