Intertwin birth weight discordance as a potential adaptive measure to promote gestational age

To determine if discordant twins are delivered at a more advanced gestational age than concordant twins of the same total twin birth weight.

We used the U.S. National Center for Health Statistics matched multiple birth data file containing all twin births in the United States from 1995 through 1997. Birth weight discordance was defined as > 25% difference between the heavier and lighter twin. We compared mean gestational age between concordant and discordant pairs at 250-g total birth weight intervals in the entire population and in the subgroups of primiparas and multiparas.

The frequency of discordant pairs declined with increasing total twin birth weight, from 7.2% at 3,000-3,250 g, to 3% at 4,750-5,000 g (R2 = .94, P < .05), with no significant difference between primiparas and multiparas (all P values > .05). The mean gestational age of discordant pairs was significantly higher across the entire range of total birth weight intervals except for the uppermost interval (> 4,750 g). The effect of discordance on gestational age was modified by parity, with significant differences between concordant and discordant pairs among primiparas mainly at the lower birth weight strata (P < .05).

In the total twin birth weight range of 3,000-5,000 g, birth weight discordant twins are delivered at a more advanced gestational age than concordant twins. Discordant growth may serve as an adaptive measure to promote maturity by reducing the inevitable uterine overdistension.