Reversal of changes in distribution of gestational age and birth weight among firstborn infants of Cardiff residents.

Firstborn singleton infants born to primiparous Cardiff residents during the decade 1970-9 were studied. Both mean birth weight and gestational age at delivery rose during the period. Changes in maternal age, height, smoking habits, and history of spontaneous or induced abortion did not explain these findings. The decreased proportion of infants weighing less than 2500 g may be explained by the overall increase in gestational age, which in turn may have resulted from decreased use of induction of labour during 1975-9.