Obesity among pregnant women may adversely affect both maternal iron status throughout pregnancy and placental transfer of iron. The objective of this study was to determine the association of maternal body mass index (BMI) with (1) maternal iron status and inflammation in mid and late pregnancy, (2) the change in maternal iron status throughout pregnancy and (3) neonatal iron status. We examined longitudinal data from 1613 participants in a pregnancy iron supplementation trial in rural China. Women with uncomplicated singleton pregnancies were enrolled in the early second trimester of pregnancy and followed through parturition. Maternal blood samples obtained at enrollment and in the third trimester and cord blood samples were analyzed for a range of hematological and iron biomarkers. There was a negative association between maternal BMI and iron status at enrollment (transferrin receptor (sTfR): r=0.20, P<0.001; body iron (BI): r=-0.05; P=0.03). This association was markedly stronger among obese women. Maternal BMI was positively associated with maternal inflammation (C-reactive protein: r=0.33, P<0.001). In multiple linear regression models, maternal BMI was negatively associated with neonatal iron status (cord serum ferritin: -0.01, P=0.008; BI: -0.06, P=0.006) and associated with a lower decrease in iron status throughout pregnancy (sTfR: -4.6, P<0.001; BI: 1.1, P=0.004). Maternal obesity during pregnancy may adversely affect both maternal and neonatal iron status, potentially through inflammatory pathways.