Calcium Supplementation in the Third Trimester of Pregnancy
Prescribe 1,000-1,500 mg elemental calcium daily as calcium carbonate, divided into 2-3 doses of no more than 500 mg elemental calcium per dose, taken with meals from the earliest antenatal contact through delivery. 1, 2
Recommended Dosage and Formulation
The WHO strongly recommends 1.5-2.0 g elemental calcium daily for pregnant women in populations with low dietary calcium intake (<900 mg/day) to prevent preeclampsia, which is a leading cause of maternal mortality globally. 1
For practical implementation, adopt doses between 1.0-1.5 g elemental calcium daily, depending on local dietary calcium intake patterns and feasibility. 1
Use calcium carbonate as the preferred formulation because it contains 40% elemental calcium by weight (compared to only 21% in calcium citrate), making it more cost-effective and requiring fewer pills. 2, 3
Divide the total daily dose into 2-3 administrations of no more than 500 mg elemental calcium per dose to optimize absorption, as fractional calcium absorption decreases significantly with larger single doses. 1, 2, 3
Timing and Administration
Calcium carbonate must be taken with meals because it requires gastric acid for optimal absorption. 2, 3
Start supplementation at the first antenatal contact, regardless of gestational age, and continue until delivery—earlier initiation may provide additional benefit by modulating placental vascularization. 1, 2
In the third trimester specifically, calcium transfer to the fetus peaks at 300-350 mg/day during the final 6 weeks, making adequate supplementation particularly critical during this period. 4
Co-Administration with Iron and Folic Acid
Instruct patients to take calcium supplements together with iron-folic acid (IFA) supplements at the same time to simplify the regimen and improve adherence. 1, 5
While calcium acutely inhibits iron absorption in laboratory studies, this translates to minimal clinical effects in real-world practice, and the practical benefit of simplified regimens far outweighs theoretical concerns. 1, 5
Do not counsel women that calcium and IFA pills must be taken separately—this creates an unnecessary barrier to adherence. 1
Clinical Benefits and Outcomes
Calcium supplementation at these doses reduces preeclampsia risk by approximately 50% in populations with low dietary calcium intake, which includes most low- and middle-income countries. 2, 6, 7
Additional benefits include reduced risk of preterm birth, longer pregnancy duration, higher neonatal birth weight, and improved maternal bone mineral density postpartum. 2, 7
These benefits are strongest with high-dose regimens (1.5-2 g/day) when split into smaller doses throughout the day. 7
Special Considerations for Calcium Citrate
Reserve calcium citrate for patients with achlorhydria, those taking proton pump inhibitors, or women who cannot reliably take supplements with meals. 2
If calcium citrate is used, prescribe approximately 7.1 g total calcium citrate daily (to deliver 1.5 g elemental calcium) or 9.5 g daily (to deliver 2.0 g elemental calcium), divided into 3-4 doses of ≤2.4 g calcium citrate per dose. 2
Calcium citrate can be taken with or without food, which is its primary clinical advantage over calcium carbonate. 2, 3
Common Pitfalls to Avoid
Do not prescribe the entire daily calcium dose as a single administration—this results in poor absorption and reduced clinical benefit. 2, 3
Do not exceed the upper safety limit of 2,000-2,500 mg elemental calcium daily, as this increases risk of cardiovascular complications and kidney stones. 3
Constipation and abdominal bloating are common side effects; counsel patients accordingly and consider stool softeners if needed. 3
In women with adequate dietary calcium intake (≥900 mg/day from diet), supplementation may not be necessary beyond standard prenatal vitamins containing 200-300 mg calcium. 8, 6
Dietary Calcium Context
The recommended total daily calcium intake for pregnant women over age 19 is 1,000-1,200 mg from all sources (diet plus supplements). 5, 8
Average dietary calcium intake in young women in Western countries is approximately 800 mg daily, leaving a gap of 200-400 mg that should be met through supplementation. 8
During pregnancy, intestinal calcium absorption efficiency doubles to help meet fetal demands, but this physiologic adaptation alone is insufficient in populations with low baseline intake. 9, 4