Abstract
Observational studies suggest that high iron intake during pregnancy is associated with the risk of gestational diabetes. As such studies are prone to bias, we re-analysed data from a randomised controlled trial of iron supplementation to see whether it supports the risk found in observational studies. The trial was conducted in primary health care setting in five municipalities in Finland in 1985-1986. The participants were 2944 women (95% of pregnant women in the area) who were randomly allocated either to (1) the selective iron group (elemental iron 50 mg twice a day only if diagnosed as anaemic, continuing until their haemoglobin increased to 110 g L(-1)) or (2) the routine iron group (elemental iron 100 mg day(-1) throughout the pregnancy regardless of haemoglobin level). The numbers of women in the analyses were 1358 and 1336, respectively. The main outcome measure was a composite variable including any glucose intolerance-related outcome (e.g. glucosuria, gestational diabetes, large-for-gestational-age child) in mothers' or children's patient records during pregnancy and post-partum. There were no statistically significant differences in the incidence of the primary outcome between the selective iron and the routine iron groups (13.0 vs. 11.0%, P = 0.12). The most common outcome was large-for-gestational-age calculated from children's hospital data (8.3 vs. 8.2%, P = 0.95). The results were mainly similar when stratified by the mothers' baseline haemoglobin level, body mass index or gestational weight gain. Routine iron supplementation throughout pregnancy did not increase the risk of glucose intolerance during pregnancy. The results need to be confirmed in future trials.