What is the physiology of calcium during pregnancy?

From the Guidelines

Pregnant women require 1000-1300 mg of calcium daily, and supplementation is recommended for those with inadequate dietary intake, with a daily dose of 1200-1500 mg in divided doses, including dietary intake, as suggested by the most recent guidelines 1. Calcium metabolism undergoes significant changes during pregnancy to support both maternal health and fetal development. During pregnancy, maternal calcium absorption increases by approximately 50%, primarily in the second and third trimesters, to meet the growing fetus's calcium demands of about 25-30 grams for skeletal development. This increased absorption is facilitated by elevated levels of 1,25-dihydroxyvitamin D, which enhances intestinal calcium absorption. Some key points to consider:

• Parathyroid hormone (PTH) levels typically decrease in early pregnancy but may rise later if maternal calcium intake is insufficient.
• The placenta also produces parathyroid hormone-related protein (PTHrP), which helps maintain the calcium gradient between maternal and fetal circulation.
• Calcium supplementation of 1000-2000 mg daily, divided into smaller doses with meals, may be necessary for women at risk of deficiency, as seen in studies 1.
• Calcium carbonate (40% elemental calcium) and calcium citrate (21% elemental calcium) are common supplement forms, with the latter being better absorbed on an empty stomach. Adequate calcium during pregnancy not only supports fetal skeletal development but also helps prevent maternal bone loss and may reduce the risk of pregnancy-induced hypertension and preeclampsia by maintaining proper vascular function and blood pressure regulation, as supported by research on calcium supplementation 1.

From the Research

Calcium Metabolism During Pregnancy

• The demand for additional calcium during pregnancy is recognized, but the dietary reference intake for calcium was lowered for pregnant women in 1997 to amounts recommended for nonpregnant women (1,000 mg/day) 2.
• Women who chronically consume suboptimal amounts of calcium (<500 mg/day) may be at risk for increased bone loss during pregnancy 2.
• Women who begin pregnancy with adequate intake may not need additional calcium, but women with suboptimal intakes (<500 mg) may need additional amounts to meet both maternal and fetal bone requirements 2.

Calcium Supplementation and Pre-eclampsia Prevention

• Calcium supplementation reduces the risk of pre-eclampsia, and both low-dose (<1 g/day) and high-dose (≥1 g/day) calcium supplementation are effective for pre-eclampsia prevention in women with low calcium intake 3.
• A network meta-analysis of 25 trials found that low-dose calcium supplementation is as effective as high-dose calcium supplementation in halving the risk of pre-eclampsia when baseline calcium intake is low 3.
• Calcium supplementation may be used for prevention of preeclampsia, and vitamin D might also work well, but further large-scale RCTs are warranted to confirm these findings 4.

Effects of Calcium and Vitamin D Supplementation on Pregnancy Outcomes

• Vitamin D plus calcium administration for 12 weeks had beneficial effects on glycaemic status, HDL-cholesterol, and blood pressure among women at risk for pre-eclampsia 5.
• Calcium and vitamin D supplementation can lower the risk of preeclampsia when compared to placebo, with pooled risk ratios of 0.54 and 0.47, respectively 4.
• The efficacy of supplementation, ranked by surface under cumulative ranking probabilities, were: vitamin D (47.4%), calcium (31.6%), and calcium plus vitamin D (19.6%), respectively 4.

Safety of Calcium and Vitamin D Supplementation

• Oral intake of calcium and vitamin D for 1 year has no effect on the urinary calcium excretion rate and the formation of kidney calculi in postmenopausal women 6.
• Asymptomatic lithiasis was confirmed in only 1 of 53 patients (1.9%) using ultrasonographic examination after 1 year of treatment with supplemental calcium and vitamin D 6.