A birthweight-for-gestation standard based on 4737 twins born in Sweden between 1983 and 1985

To construct a birthweight-for-gestation chart based on a population-based material of twins born during a restricted period of time.

Nearly all live-born twins born in Sweden between 1983 and 1985, 4737 all told, were included. Excluded from the analysis were 4.7% of the twins that were not on record at the Medical Birth Registry, 1.8% that died before or during birth, and 8.8% that were without a recorded date of birth or only had information on gestational duration estimated from the first day of the last menstrual period.

The standard deviation in birthweight was considerable in most gestational weeks; the range (+/- 2 SD) was 1000-3400 g even if exclusively twins (pregnancies) with estimated date of birth from ultrasound examination early in the second trimester were included. Intra-pair discordance increased with gestational duration, especially for unlike-sexed twins, to reach over 400 g at term. On average, boys weighed 92 g more than girls and twins to multiparae weighed 250 g more than twins to primiparae. Maternal age affected birthweight for twins to primiparae, but not to multiparae.

Although the problems with secular trends in birthweight were avoided by using a population-based material during a restricted time period, and only twin pregnancies dated with ultrasound were included, the standard deviation in most gestational weeks was still considerable, reflecting the multiplicity of factors involved in determining the birthweight of twins.