Objectives: To determine the demographic, environmental, and medical factors that influence the relative weights of the newborn infant and the placenta and compare this ratio with other factors known to predispose to adult ill health. Design: Prospective cohort study. Setting: The tertiary referral centre for perinatal care in Perth, Western Australia. Subjects: 2507 pregnant women who delivered a single live infant at term. Main outcome measures: Placental weight, birth weight, and the ratio of placental weight to birth weight. Results: By multiple regression analysis the placental weight to birthweight ratio was significantly and positively associated with gestational age, female sex, Asian parentage, increasing maternal body mass index, increased maternal weight at booking, lower socioeconomic status, maternal anaemia, and increasing number of cigarettes smoked daily. There were no consistent relations between the placental weight to birthweight ratio and measures of newborn size. Conclusions: The ratio of placental weight to birth weight is not an accurate marker of fetal growth. In its role as a predictor of adult disease the ratio may be acting as a surrogate for other factors which are already known to influence health and may act before or after birth. Determining the role that relative growth rates of the fetus and placenta have in predisposing to adult disease requires prospective study to account for the many confounding variables which complicate this hypothesis. Retrospective analyses have identified an association between a raised placental weight to birthweight ratio and hypertension in adulthood Accurate estimation of gestational age is crucial when interpreting the placental weight to birthweight ratio Environmental factors associated with alterations in the placental weight to birthweight ratio may not exert their effects exclusively in the antenatal period As a marker of fetal growth the potential usefulness of the placental weight to birthweight ratio is diminished because the ratio is influenced by a multiplicity of factors Prospective study is required to clarify the role of intrauterine programming in the genesis of adult disease