Birth weight is related to neonatal health and long-term risk of chronic disease. Since animal studies have shown that birth outcome is related to placental function, the present project was designed to explore the relationship between birth weight and placental growth and composition with maternal factors during pregnancy among normal term pregnancies in 51 primiparous and 40 multiparous women delivering at the University Hospital of the West Indies. Both groups were followed from 15 weeks of gestation to term. The primiparous group was generally younger than the multiparous (mean age 22 +/- 4 versus 31 +/- 5 yr). They were significantly lighter (55 +/- 8 versus 61 +/- 9 kg) with a lower body mass index (21 +/- 3 versus 23 +/- 4 kg/m2) during early pregnancy, but gained more weight during pregnancy, 11 kg compared with 8 kg, respectively. The duration of pregnancy was similar for both groups. Although the size of the placenta was not significantly different between the two groups, the mean weight of the multiparous placentae was more than that of the primiparous placentae. Also, for all mothers both placental weight and initial maternal weight related directly to birth weight. Placental non collagen protein (NCP), sodium and potassium contents were significantly higher for multiparous women and were related to birth weight. The primiparous group had babies who were significantly lighter, 3.03 kg compared with 3.36 kg, for the multiparous and this could be attributed to differences in placental function and maternal weight. When account was taken of the difference in maternal weight at the start of pregnancy and the difference in placental weight, parity no longer explained any of the differences in birth weight. It is concluded that maternal body weight at the time of becoming pregnant and the early development of the placenta determine the efficiency with which nutrients might be delivered to the foetus and hence foetal growth. The difference in birth weight between primiparous and multiparous women can be explained by the differences in maternal weight at the time of becoming pregnant.