Poor maternal and newborn health and nutrition remain significant contributors to the burden of disease. In 2010, 3.1 million babies died in the first 28 days of life, mostly due to low birth weight, severe infections, asphyxia and preterm birth. Every year, 15 million babies are born prematurely, of whom 1.1 million die in the neonatal period or in infancy. In addition, many of those who survive have a lifetime disability such as learning disabilities and/or visual and hearing problems (2).
Approximately 287 000 women died during pregnancy and childbirth in 2010, mostly due to maternal health complications (3). Hypertensive disorders of pregnancy include (pre-existing) chronic hypertension and gestational hypertension, pre-eclampsia and eclampsia (4,5). These disorders complicate approximately 2-8% of all pregnancies and have been associated with preterm and low birth weight and maternal mortality (4). Pre-eclampsia is diagnosed when gestational hypertension (maternal blood pressure ≥140/90 mmHg for the first time in the second half of pregnancy) is accompanied by proteinuria >300 mg in a 24-hour period. The pathogenesis of pre-eclampsia has not been thoroughly elucidated, however, it is related to disturbances in placentation in early pregnancy, followed by generalized inflammation and progressive endothelial damage. Pre-eclampsia can be classified as mild or severe. In severe pre-eclampsia, the blood pressure is ≥160/110 mmHg and there is proteinuria ≥2 g/24 h, with or without substantial maternal organ damage (5). Such end-organ damage as a result of pre-eclampsia can present with haemolysis, elevated liver enzymes and low platelet count, a constellation of symptoms known as HELLP syndrome (5). This is a rare condition that occurs in 10–20% of cases with severe pre-eclampsia (6).
Calcium is the most abundant mineral in the body and is essential for many diverse processes, including bone formation, muscle contraction, and enzyme and hormone functioning (7). Most of the body's calcium is found in the bones and teeth; approximately 1% is present in the intracellular structures, cell membrane and extracellular fluids (8). Calcium absorption increases during pregnancy and no additional intake is needed (9). A dietary intake of 1200 mg/day of calcium for pregnant women is recommended by WHO and the Food and Agriculture Organization of the United Nations (FAO) (7). Inadequate consumption of this nutrient by pregnant women can lead to adverse effects in both the mother and the fetus, including osteopenia, tremor, paraesthesia, muscle cramping, tetanus, delayed fetal growth, low birth weight and poor fetal mineralization (10).
Serum calcium concentrations are maintained within narrow limits in the body and thus have limited use for the assessment of calcium nutritional status at both the individual and the population levels. Calcium intake could be a useful indicator of status at the population level. The main dietary sources of this nutrient are milk, dairy products, calcium-set tofu and fortified foods (9); some local foods such as lime-treated corn meal also have abundant calcium. There is no information on the adequacy of calcium intake worldwide. However, some studies at the regional and national levels suggest that low calcium intake at population level occurs frequently (11, 12).
Various studies have suggested that calcium supplementation during pregnancy has a beneficial effect on reducing the risk of pregnancy-induced hypertension (10). The results of trials evaluating the effect of supplementation on maternal bone mineral density, fetal mineralization, and preterm birth, however, are less conclusive (13). Excessive consumption of calcium may increase the risk of urinary stones and urinary tract infection, and reduce the absorption of other essential micronutrients (13).
In supplements, calcium is present in the form of carbonate, citrate, lactate or gluconate, and in general all these forms have good bioavailability (14, 15). At least one salt of calcium for oral administration (in a variety of doses) is included in most national essential medicines lists (16), calcium carbonate being the most common. As calcium carbonate has the highest content of elemental calcium (40%), it may have the best efficacy-cost ratio in pregnancy (17), but this needs to be confirmed in future analyses.
