Second-breech presentation in twins--a possible adaptive measure to promote fetal growth.

Two hundred eighty-three longitudinally lying twin pairs were evaluated for the possible etiology of breech presentation in twins. Breech-breech pairs were delivered at a significantly earlier gestational age (P less than .02), because of significantly more deliveries (P less than .04) between 30-32 weeks, compared with vertex-second twins. Nevertheless, breech-breech pairs attained similar or larger birth weights when compared with other combinations. Discordant-second twins were found five times more frequently in breech-second pairs when twin A was in the vertex presentation. It is possible that breech-breech pairs reach a critical utero-fetal proportion earlier, resulting in earlier labor. Alternatively, twins may arrange in second-breech presentations as an adaptive measure to promote growth. We recommend special care for persistent breech-breech presentations diagnosed at the beginning of the third trimester.