Calcium is an essential nutrient for everyone, but it’s particularly important for pregnant women. It builds strong bones and carries out other essential functions, including helping to form a baby’s teeth and skeleton, developing a healthy heart rhythm, clotting blood, and keeping muscles and nerves working properly. A deficiency of calcium can also reduce a woman’s bone density and increase her risk of osteoporosis later in life.
The demands on maternal calcium during pregnancy are high, and adequate intake is important for both the fetus and the mother. A fetus without sufficient calcium is at risk of restricted intrauterine growth, low birth weight, poor bone development, and an increased risk of preeclampsia and hypertension. If a woman does not consume enough calcium, her body will take it from her own bones, a process known as “osteoporosis in pregnancy”. A woman’s risk of osteoporosis increases with every extra birth she has, so limiting the number of pregnancies is a good way to manage this risk.
A recent Cochrane review of 13 randomized controlled trials found that a supplementation regimen of at least 1 g of daily calcium beginning in mid-pregnancy (20 weeks) was associated with a 55% reduction in the risk of developing preeclampsia. The effect was more pronounced in women at high risk of preeclampsia (defined as those who had experienced eclampsia in a previous pregnancy or those with low dietary calcium intake) and was not influenced by gestational age, race/ethnicity, or parity.
However, the authors of this study note that calcium intake was measured only in a small subgroup of participants and that the results were slightly over-estimated due to the small sample sizes used for analysis. A nested study within the large WHO trial showed no effect of calcium supplementation on biochemical markers commonly elevated in pre-eclampsia, such as proteinuria and serum urea nitrogen (SUN).
Another limitation of this study is that the rate of loss to follow-up was quite high (53% at 3 months and 90% at 2 years), so the findings cannot be generalized. It is also not clear whether the participants were consuming enough calcium through food sources and/or prenatal supplements, although they reported their intake in a well-designed semi-quantitative food frequency questionnaire. In addition, the analyses used data on both systolic and diastolic blood pressure, which may have introduced some confounding effects. This is an important area for future research.