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

We studied data on firstborn singleton infants born to primiparous Cardiff residents during the decade 1965-1974. Both mean birth weight and gestational age at delivery fell appreciably during 1965-74. Changes in maternal age, height, smoking habits, or history of abortion did not explain these findings. The increased proportion of infants weighing less than 2500 g may be explained by the overall reduction in gestational age at delivery, which, in turn, may have resulted from increased use of elective delivery during the second quinquennium.