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Abstract

Our article described extreme birth weight discordance in <32-week gestations as an epidemiologic risk factor for preterm birth. We appreciate the letters indicating that these deliveries may have been iatrogenic, because that point was not made adequately clear in the article. Abdominal delivery is often performed because of birth weight discordance in the absence of labor, even at very preterm gestation. Information about cesarean delivery is available on the birth certificate but was not reported in the article, because this variable may be underreported and its reliability has not been established. As reported in the birth certificates in our study, abdominal delivery was performed in 26 of 42 births at <32 weeks’ gestation with ≥40% discordance (60.5%) and in 425 of 929 births at <32 weeks with <40% discordance (44.9%; relative risk, 1.35; P =.06) We regard this information as inconclusive regarding the question of whether the mechanism of preterm births in highly discordant twins was physiologic or iatrogenic. Further, we do not know why the association was so clearly limited only to preterm births at <32 weeks’ gestation. Nor do we know whether those with iatrogenic deliveries would otherwise soon have had spontaneous labor because of fetal distress or fetal death. The purpose of our studies of preterm birth was to provide epidemiologic information relevant to possible mechanisms of preterm birth. In that sense perhaps the most useful information in our data was not the association of discordance with preterm birth but rather how weak the association was. Essentially, the association was seen only for preterm birth at <32 weeks’ gestation, and even then only for rather extreme discordance. Only 42 of 971 twin preterm births <32 weeks’ gestation (4.3%) demonstrated ≥40% birth weight discordance, which appears to indicate that even severe growth failure has relatively little importance as a cause of preterm birth in twin gestations, whether the mechanism is iatrogenic or physiologic. 6/8/112867