National birthweight percentiles by gestational age for twins born in Australia

Objective: To develop national charts of birthweight percentiles by gestati onal age and infant sex for liveborn twins born in Australia. Methodology: National data on live twin births to non-indigenous Australian -born mothers during 1991-94 were derived from perinatal data collected by midwives in each State and Territory. Results: During 1991-4 there were 20 075 liveborn twin infants. Of these bi rths, missing data included: birthweight 36 (0.2%) births, gestational age 95 (0.5%) births, and sex (missing or indeterminate) 13 (0.06%) births. The se births were excluded from the study. An additional 0.6% births were excl uded because the recorded birthweights were extreme outliers for the record ed gestational ages. Forty-seven per cent of live twin births were preterm (< 37 weeks). At all gestational ages, the median birthweight of male twins was higher than that of female twins. At modal gestational age of 38 weeks , the difference in the median birthweight was 130 g. Conclusions: The charts produced as a result of the study provide birthweig ht percentiles by gestational age for twins based on national data in Austr alia. They provide current population norms for the use of Australian clini cians and researchers.