Gestational age-specific distribution of twin birth weight discordance

To examine the gestational age-specific distribution of twin birth weight discordance.

We analyzed all liveborn twin sets between 28 and 40 weeks' gestation from the United States 1995-1998 Multiple Matched Birth Data Set compiled by the National Center for Health Statistics. We calculated the 50th and 95th percentiles of birth weight discordance at each gestational age. Neonatal mortality rates were calculated for discordant twins at the 95th percentile of birth weight discordance for each gestational age.

At older gestational ages, the 95th percentile of birth weight discordance resulted in an inter-twin birth weight difference of approximately 25%, a value often used to define twins as birth weight discordant. However, at earlier gestational ages, the 95th percentile of birth weight discordance was greater, reaching nearly 50% at 28 weeks.

The inter-twin birth weight difference at the 95th percentile is greater at lower gestational ages, possibly illustrating the different nature or severity of twin birth weight discordance at an earlier gestational age.