Birth weight and head circumference for 22-29 weeks gestation neonates from an international cohort.

Objective: Size at birth is a key indicator of in utero growth. Our objective was to generate sex-specific percentiles for birth weight and head circumference in neonates born between 22 and 29 weeks gestation from pregnancies without hypertension or diabetes and assess differences between vaginal and caesarean births and between singletons and twins.

Methods: We used data from 12 countries participating in the International Network for Evaluating Outcomes in Neonates database from 2007 to 2021. We excluded data that were influenced by truncation with 1500g birth weight cut-offs in databases and neonates with major congenital anomalies or born to mothers with hypertension or diabetes.

Results: After exclusions, 132 727 neonates contributed to birth weight and 65 406 contributed to head circumference. The percentiles of birth weight were similar between countries at the 50th and 90th percentiles, though variability was noted in the lower percentiles from countries with smaller sample sizes. Head circumference percentiles were comparable between countries. Caesarean births had birth weights similar to vaginal births until 26 weeks after which the weight at 10th percentile diverged by approximately 239 g at 29 weeks. Vaginal births had birth weights very similar to Hadlock's intrauterine estimated fetal weights. There were no differences in head circumference between vaginal and caesarean births and between singletons and twins.

Conclusions: We present updated information on weight and head circumference at birth for preterm neonates of 22-29 weeks gestation born to mothers without hypertension or diabetes derived from a large multicountry cohort. Research is needed to explore the predictive value of these birth size data for health and developmental outcomes.