Adequate calcium intake during pregnancy is of major importance for the health of both mother and fetus. However, the likelihood of pregnant women achieving recommended intakes is low albeit evidence in high income countries is limited.

For example, dietary survey studies of pregnant women:

• Netherlands: 42% below the EAR of 800 mg/d (2020)
• US: 13% below EAR of 800 mg/d (2019)

A greater number of studies indicate a substantial proportion of inadequate calcium intakes among reproductive age women.

While pregnant women are often more motivated to consume a 'healthier' diet – their ability to consistently do so compared to the pre-pregnancy period can be challenging – and so, the risk of inadequate calcium intakes throughout pregnancy remains. 

Recommended dietary calcium intakes for pregnant women range across countries and various authorities from 900 to 1300 mg per day.

• US and Canada – RNI 1000 mg per day
• NZ and Australia – RDI 1000 mg per day

Interestingly, many authorities have not increased intakes beyond the nonpregnant state due to compensatory changes in calcium homeostasis during pregnancy. These changes include increased calcium gut absorption, bone resorption (break down of bone tissue and release of minerals) and kidney tubular reabsorption of calcium.

Beneficial effects of calcium during pregnancy

Insufficient calcium intake during pregnancy has been linked to the development of maternal hypertension, an important factor of maternal morbidity (death), fetal growth restriction (small-for-gestational age infants) and preterm births.

Calcium supplementation from randomised controlled trials show that calcium supplements help prevent or lower the risk of:

• pre-eclampsia (high blood pressure and urinary protein)
• preterm birth
• small-for-gestational age infants
• lowers the risk of a woman dying or having serious problems related to high blood pressure in pregnancy

These risks are particularly relevant for women with low calcium diets – often defined as less than 500-600 mg calcium per day
 
Pre-eclampsia is a major cause of death in pregnant women and newborn babies worldwide. Women at risk of preeclampsia include one or more of the following factors: obesity, previous pre-eclampsia, diabetes, chronic hypertension, renal disease, autoimmune disease, nulliparity, advanced maternal age, adolescent pregnancy and conditions leading to hyperplacentation and large placentas (e.g. twin pregnancy)
 
Preterm birth (birth before 37 weeks) is often caused by high blood pressure and is the leading cause of newborn deaths, particularly in low-income countries.
 
Cochrane Reviews

The most recent Cochrane review published in 2018 supports the randomised controlled findings as follows:

• High-quality evidence from 13 studies (involving 15,730 women) that high dose calcium supplementation (at least 1 gram (g) daily) during pregnancy may be a safe way of reducing the risk of pre-eclampsia, especially in women from communities with low dietary calcium and those at increased risk of pre-eclampsia.
• Limited evidence from 12 trials (2334 women) suggested that a relatively low dose of calcium may be effective in reducing pre-eclampsia, high blood pressure, and babies admitted to intensive care. However, the quality of the evidence on calcium alone was reduced because eight of the included trials gave other medicines alongside calcium, such as vitamin D, linoleic acid or antioxidants.
• Low quality evidence that women receiving calcium supplements may also be less likely to die or have serious problems related to pre-eclampsia and high blood pressure.
• Low quality evidence that babies may be less likely to be born preterm

Ideal Dosage
 
In 2013, the WHO published recommendations for calcium supplementation as part of antenatal care for women with an inadequate dietary calcium intake to lower the risk of developing preeclampsia.

The suggested scheme for calcium supplementation below:


 
In 2018, the WHO re-validated the above recommendation with a particular aim to investigate whether supplementation prior to pregnancy and/or early pregnancy conferred additional benefits.

The evidence, at the time, was insufficient to pinpoint at what gestational age calcium supplementation should be started.

Nonetheless, the WHO did recommend that calcium supplementation should commence at the first antenatal visit in order to optimise compliance with this regimen. 

Overall, further research is needed into the ideal dosage of supplementation.

Where high-dose supplementation is not feasible, the option of lower dose supplements (500 to 600 mg daily) might be considered in preference to no supplementation.

A reminder that vitamin D is essential for calcium and bone metabolism, and poor vitamin D status may impair calcium absorption and utilisation. Currently the WHO does not recommend vitamin D supplementation for pregnant women due to lack of data on the effects of vitamin D on bone health in the infant along with little knowledge on supplement initiation, dose and duration.

Bottom Line

• High dose calcium supplementation (between 1500 - 2000 mg daily) reduces the risk of pre-eclampsia, especially in women with low dietary calcium and those at increased risk of pre-eclampsia.
• Where consumption of calcium meets the recommended intake and risk of preeclampsia is not evident, supplementation is NOT encouraged.
• Supplementation requires monitoring of women's total daily calcium intake (diet, supplements and antacids). Overall daily intake should not exceed 3 g/d.

Next up - the how-to...

Best Practice

Calcium-Rich Foods and Supplements

Adequate calcium intake can be easily achieved by the incorporating dairy products or calcium-fortified foods in the diet on a daily basis.

NZ Health Navigator lists the calcium content of foods including a helpful table on 'How to Eat About 1000 mg of Calcium in a Day' with and without dairy products.

Calcium supplements can be recommended for those with low dietary intake and women at risk of preeclampsia (obese, diabetic, advanced age).

The acceptability of calcium supplements may vary – as most tablets can be large, have a powdery texture and be unpalatable to consume.

The most common forms of calcium supplements are: calcium carbonate (40% elemental calcium); calcium citrate (21% elemental calcium); calcium lactate (13% elemental calcium); and calcium gluconate (9% elemental calcium).

Note that elemental calcium is what the body absorbs. For example, 1000 mg of calcium carbonate contains 400 mg of elemental calcium (40%).

Calcium supplements should be taken in small doses (500 mg at a time) and preferably at mealtime to increase absorption.

Possible side effects include gastrointestinal upset, such as bloating, gas, and constipation.

Nutrient and Drug Interactions for:

• Those taking iron supplements: Iron and calcium interact such that the two nutrients should be preferably administrated several hours apart
• Those taking synthetic thyroid: Calcium supplements can interfere with the absorption of thyroid hormones, so again a time gap between these two pills is needed.
• Those taking antacids: Antacids like Tums contain calcium and caution should be exercised when supplementing with calcium