Placental function in early pregnancy, as measured by circulating concentrations of placentally derived proteins in the mother's serum, is associated with risk of low birth weight.4 Moreover, animal models suggest that periconceptional undernutrition affects the timing of birth and, hence, birth weight.5 A single study has shown that fetuses that were smaller than expected in the first trimester (on the basis of menstrual history) were at increased risk of being low birth weight.6 This has been interpreted as indicating that impairment of fetal growth begins in the first trimester. However, when the expected size of the fetus is based on menstrual history, a smaller than expected fetus in early pregnancy may also reflect delayed ovulation. Other authors have suggested that a prolonged menstruation-conception interval is associated with shorter duration of pregnancy and lower birth weight.7 The uncertainty in the interpretation of existing studies relates to the inherent uncertainty in using menstrual history to estimate the date of conception. We assessed the relation between discrepancy in the observed and expected size of the fetus in the first trimester and the birth weight, duration of pregnancy, and risk of delivering a small for gestational age infant among a large cohort of women with a known date of conception.