• birthweight;
  • cesarean sections;
  • gestational age;
  • preterm births;
  • stillbirths

Objective. To describe birthweight by gestational age in Norway for the period 1967–1998, evaluate secular trends and provide new standards for small for gestational age for 16 to 44 weeks of gestation.

Subjects and methods. The analyses were based on more than 1.8 million singleton births, covering all births in Norway for a 32 year period. Percentiles for birthweight by gestational age were estimated using smoothed means and standard deviations. In the preterm weeks, means and standard deviations were carefully screened for birthweight-gestational age consistency, adapting a method of Wilcox and Russell. Differences in birthweight by gestational age for stillbirths and livebirths in extremely preterm weeks (16–28) are presented, and the effects of cesarean section are evaluated. We observed a clear increase in birthweight by gestational age for all term weeks, but a decrease for most of the preterm weeks over the same period. This decrease was related to the increase in deliveries by cesarean section.

Conclusions. Percentiles for birthweight by gestational age are presented for clinical use, based on a current period 1987–98, covering 20–44 completed gestational weeks. In the final standards we excluded stillbirths, infants born with malformations and cesarean sections. Birthweights in the Scandinavian populations are high and standards from other populations may not be representative, especially for the term weeks. Also, the secular changes demonstrated in this study indicate that old birthweight by gestational age standards need revision, especially due to changes in obstetrical routines influencing preterm data.