Birth weight and ponderal index percentiles for twins based on sex and chorionicity in a center of Guangdong Province, China

Background: Intrauterine growth restriction (IUGR) which is judged based on birth weight and gestational age, is associated with increased neonatal mobility and mortality and also has a further impact on physical and mental health during later in life. Using the birth weight percentile for singletons to assess twins might not accurately reflect the growth status of the twins; this could potentially lead to an incorrect evaluation of growth-restricted children. For a more precise assessment of twin newborns, it is beneficial to utilize twin-specific birth weight percentile curves and ponderal index (PI) curves that consider factors such as birth order and sex. The aim of this study is to establish a contemporary up-to-date method that provides sex-specific percentiles for birth weight and the PI of twin neonates, categorized by gestational age and differentiated by chorionicity within a defined cross-sectional study.

Methods: We retrospectively analyzed the birth weight and PI percentile of 3,433 twins born at a gestational age between 25 and 40 weeks based on sex and chorionicity, between 2015 and 2020 in a single center in southern China. The smoothed percentile curves were drawn via generalized additive models for location, scale and shape (GAMLSS).

Results: A total of 3,433 live-born twins were included in this study and consisted of 366 monochorionic girls, 405 monochorionic boys, 1,237 dichorionic girls and 1,425 dichorionic boys. Compared with dichorionic twins, the mean birth weight of monochorionic twins was significantly lower and the gestational week of delivery was earlier. The calculated 50th percentile birth weight values for twin boys were higher than those for girls based on the same chorionicity, except for those male monochorionic twins who were born before week 31 of gestation.

Conclusions: The birth weight and PI percentile charts for neonates, categorized by sex and chorionicity, serve as valuable tools for assessing the wellbeing of twin infants. Utilizing these charts could reduce the overdiagnosis of small for gestational (SGA) age in normally growing twins.