International Normative Charts for Twin Weight, Length, and Head Circumference at Birth, By Gestational Age and Sex: The Twin Neonatal Cross-Sectional Study of the INTERGROWTH-21st Project.

Assessing the size of twins at birth using charts developed for singletons may over diagnose small for gestational age in this sub-population. The study aimed to produce international, twin-specific, newborn size normative charts by gestational age and sex. This longitudinal observational study in eight geographically diverse settings prospectively collected data between May 2009 and August 2013 from healthy pregnant women and their newborn twins. The participants were enrolled as part of the INTERGROWTH-21st study, and recruited based on World Health Organization recommendations for evaluation of anthropometric measures. All the women met, in addition to the underlying population characteristics of low perinatal risk, strict individual criteria for a population at low risk of impaired fetal growth. Newborn weight, length and head circumference measures were collected independently in duplicate by two trained anthropometrists within 12 hours of birth using identical equipment and protocols at all sites. From 1034 multiple pregnancies, after exclusions of condition such as smoking, high maternal BMI, and congenital malformations, the final sample was 864 twin newborns. Most of the twins were below the 50th centile of the INTERGROWTH-21st standards for singletons. We present international newborn size normative charts for twins using the same methodological approach adopted to construct the singleton standards.