Zinc is an essential mineral known to be important for many biological functions including protein synthesis, cellular division and nucleic acid metabolism. Severe zinc deficiency is rare in humans, but mild to moderate deficiency may be common, especially in populations with low consumption of zinc-rich animal-source foods and high intakes of foods rich in phytates, which inhibit zinc absorption. It is estimated that over 80% of pregnant women worldwide have inadequate zinc intake, consuming on average 9.6 mg zinc per day, well below the recommended minimum daily levels for the last two trimesters of pregnancy in settings of low zinc bioavailability.
It has been suggested that maternal zinc deficiency may compromise infant development and lead to poor birth outcomes. Low plasma zinc concentrations reduce placental zinc transport and may affect the supply of zinc to the fetus. Zinc deficiency also alters circulating levels of a number of hormones associated with the onset of labour, and because zinc is essential for normal immune function, deficiency may contribute to systemic and intra-uterine infections, both major causes of pre-term birth. Low birthweight and prematurity are significant risk factors for neonatal and infant morbidity and mortality. It has been hypothesized that zinc supplementation may improve pregnancy outcomes for mothers and infants.
Studies of the effects of zinc supplementation during pregnancy have shown inconsistent results, possibly in part because of the challenges in establishing baseline zinc status in populations. Two recent systematic reviews report meta-analyses of randomised controlled trials of zinc supplementation during pregnancy conducted across five continents between 1977 and 2008, largely among women of low socioeconomic status, on a variety of maternal and neonatal outcomes. Both reviews concluded that zinc supplementation was associated with a significant reduction in preterm birth of 14% (summary relative risk (RR) 0.86 [95% CI 0.75, 0.99] and [95% CI 0.76 to 0.97]). However, zinc supplementation had no apparent effect on other infant outcomes including neonatal mortality, mean gestational age or any parameter of fetal growth such as risk of low birthweight or mean weight, length or head circumference at birth, or on primary maternal outcomes such as pre-eclampsia. The effect of zinc supplementation on premature birth may be due to a reduction in the incidence or severity of maternal infections, which are a known risk factor for premature birth.
The overall nutritional status of the mother during pregnancy is a significant contributor to both maternal and perinatal mortality and morbidity and improving the quality, or nutrient density, of the mother’s diet is known to improve pregnancy outcomes. Given the limited effect observed for zinc supplementation on pregnancy outcomes, it may be more prudent to focus research on identifying ways of improving the overall nutritional status of women in low-income areas. Currently UNICEF is promoting antenatal multiple micronutrient supplements which include zinc, iron and folic acid, for all pregnant women in developing countries, given that they are likely to have low micronutrient intakes from diet alone.9 Although there appears to be no harmful effects of zinc supplementation. The overall public health benefit of zinc supplementation in pregnancy currently appears limited.