Genetics of size at birth.

S ize 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.