The tendency to repeat gestational age and birth weight in successive births.

A study was conducted in Norway on all 454,358 single births which occurred during the 1967-73 period. The results confirm and strengthen earlier findings regarding the tendency to repeat low-birth-weight and small- or large-for-gestational age deliveries in later births. Data collected from the Medical Birth Registry included information on birth weight, length of gestation, mothers' health during pregnancy, and complications and interventions during labor and delivery. A contour level graph was prepared to illustrate the joint distribution of gestational age and birth weight. Relative risks were calculated for each type of birth. These risks were correlated with actual outcome of subsequent births. The tendency to repeat birth weight in later births was greatest for mothers of heavy infants. The tendency to repeat gestational age in subsequent births, however, was greatest for mothers with preterm infants. Delivery of prior preterm births carried a lower risk than delivery of prior low-weight births. These tendencies were unaffected by the sex of the offspring or the birth order. The pattern of repeating similar gestational age and birth weight subsequently could not be accounted for by a tendency to repeat medical complications which predispose toward such outcomes. Such environmental factors as smoking or socioeconomic status were not believed to be relevant in the study results. The study demonstrates a cumulative risk for 3rd and later pregnancies.