We previously reported that pregnant women whose plasma Zn concentrations were below the 50th percentile tended to have high pre-pregnancy BMI (kg/m(2)) values. We therefore hypothesized that in pregnant women, plasma Zn concentrations are negatively correlated with BMI. We evaluated the association between BMI values and plasma Zn concentrations in 1474 women whose blood samples were obtained before 15 weeks of gestation. Their mean age was 22.7 years and mean gestational age at blood sampling was 10 weeks. The mean plasma Zn concentration and BMI were 11.6 micromol/l and 26.6 kg/m(2) respectively. Because plasma Zn concentrations decrease as gestational age increases, plasma Zn concentrations were standardized by Z-scores. Z-score distributions were compared among the quartiles of BMI. The highest BMI group had the lowest plasma Zn concentrations, whereas the lowest BMI group had the highest; the differences were significant between the BMI groups (P<0.0001). The interpretation of plasma Zn concentrations to assess Zn nutriture in pregnancy may be complicated not only by the well-established factor of gestational age at blood sampling, but also by a previously unrecognized factor, maternal BMI.