Iron but not folic acid supplementation reduces the risk of low birthweight in pregnant women without anaemia: a case-control study.

OBJECTIVE To assess whether iron and folic acid supplementation reduce the risk of low birthweight (LBW) in women without anaemia. DESIGN Case-control study. SETTING University Hospital of Cantabria. STUDY POPULATION Cases were 322 mothers without anaemia delivering a singleton infant of less than 2500 g. Controls were 934 mothers without anaemia delivering a term non-small-for-gestational-age infant. DATA COLLECTION Data on iron and folic acid supplementation were obtained from prenatal chart record and personal interview. Data on risk factors for LBW were also gathered. RESULTS Agreement between the two sources of information was good (82% for folic acid and 94% for iron). Odds ratios yielded from the two sources were very close. Folic acid only (15 mg/day) was unrelated to LBW, whereas iron supplementation (80 mg ferrous sulphate) was associated with a lower risk of LBW (odds ratio (OR) 0.58, 95% CI 0.34 to 0.98), adjusted for smoking, maternal education, body mass index, obstetric diseases during pregnancy, weight gain during pregnancy, and previous LBW. The results of iron plus folic acid were similar to those for iron (OR 0.56, 95% CI 0.33 to 0.96). There was a significant trend towards a lower risk of LBW (p<0.001) with the duration of iron supplementation. After stratifying by the type of LBW, the trend was also significant for any kind of LBW. CONCLUSIONS Iron supplementation is associated with a lower risk of LBW in pregnant women without anaemia.