Size at birth is strongly related to a number of maternal factors including parity, length of gestation, mother's adult size, and mother's own birth weight (1). The importance of genetic factors has come from studies of monozygous and dizygous twins, where estimates of heritability of birth weight range from 30 to 70% (2,3). Ounsted et al. (4), however, reported that there may be a stronger relationship between the birth weight of the mother and that of the offspring, particularly in infants born with a low birth weight, and that these relationships may vary with parity. These data suggest that not only fetal genes but also genes that regulate the maternal uterine environment could be important in determining size at birth. Size at birth is the strongest determinant of perinatal survival (5), yet in most populations, mean birth weight is slightly lower than optimal for offspring survival (6). Thus, it would appear that all fetal growth is subject to some degree of restraint by the maternal uterine environment (7), perhaps reflecting the importance to the mother of restricting the nutritional demands of the fetus if it would threaten her survival in times of poor nutrition. Although maternal undernutrition may (8) or may not (9) be a less common determinant of birth weight in contemporary populations, restraint of fetal growth is still evident, particularly in first pregnancies. First babies have a lower birth weight and tend to be thinner than subsequent babies, with a preserved head circumference and length, suggesting reduced adiposity. Postnatally, these babies demonstrate rapid catch-up growth (1). Such a growth pattern is evident in infants whose intrauterine environment has been affected by poor placental function, secondary to maternal hypertension and preeclampsia, but a similar pattern is also evident in uncomplicated pregnancies, where it is predicted by maternal …