A new study found that it does neither, but may be helpful in preventing pregnancy-induced hypertension. The study, conducted in Thailand, examined 21 randomized controlled trials with 17,000 participating pregnant women. Lead author Dr Pranom Buppasiri (from the obstetrics and gynecology department at Khon Kaen University) and his colleagues set out to learn whether taking calcium supplements was more beneficial for pregnant women than a placebo or no pill at all.
The women who took calcium supplements had no higher bone density than those who did not. Now, one thing that comes to mind is that placebo tablets often contain calcium, but I guess that the researchers took care of that one. Before you think that taking calcium supplements in pregnancy is useless, though, the researchers did find benefits.
Women taking calcium supplements while they were expecting a baby had a slightly higher birth weight, but more importantly a pregnant woman's risk of developing the dangerous pregnancy complication preeclampsia was almost halved! Preeclampsia makes the blood pressure quickly shoot up to dangerously high levels and can be life-threatening.
That is more than enough reason to take a calcium supplement, I think. But of course, a balanced and healthy diet with plenty of naturally occurring calcium would be even better. The study was published in the Cochrane Database of Systematic Reviews, for those who would like to take a closer look at it. And, if you are interested in pregnancy nutrition and vitamin and mineral supplements, you may like to read about food sources of folic acid (folate) too.