Previous studies have found that there is a correlation between mothers' haemoglobin concentration or packed cell volume and infants' birth weight, and that iron supplementation increases mothers' haemoglobin concentration. The purpose of this study, using the data of a large randomised trial on iron prophylaxis during pregnancy, was to find out whether iron supplementation causes fetal growth to deteriorate. At their first antenatal visit, 2912 pregnant women were randomised into non-routine iron and routine iron supplementation. The mean length of gestation was shorter in the non-routine group. Birth weight did not differ between the groups, but due to longer gestations boys in the group receiving routine iron were taller than in the non-routine group. In both groups, whether studied by various values of packed cell volume or correlation coefficients, the lower the packed cell volume, the heavier and taller the infant and heavier the placenta. These negative correlations could be seen even with a packed cell volume measured early in pregnancy. Standardising for blood pressure did not influence the correlation coefficients. The correlation between a high ratio for packed cell volume and poor fetal growth thus may not be caused by iron supplementation, nor mediated by blood pressure, but by some other mechanism.