Increasing rates of preterm twin births coincide with improving twin pair survival

Objective: To examine trends in twin gestational age over time, with adjustment for potential confounding factors, and to assess twin pair mortality and respiratory support over time. Methods: Rates of preterm births, respiratory support, and neonatal mortality were calculated for 21,569 twin pairs born from 1980 to 2005 in Washington State, using birth certificate and hospital discharge data. Fetal death risks were determined on a "per-pair-at-risk" basis. Results: While the proportion of twins born at 24-31 weeks remained stable at 8%, the proportion born at 32-36 weeks increased from 28% to 48%, and the proportion at 37-42 weeks declined from 64% to 44% (P < 0.0001). Controlling individually for a variety of factors, such as maternal age, race, parity, and mode of delivery did not diminish the highly significant trend of increasing preterm births (P < 0.0001 for each). Twin pair neonatal mortality decreased significantly through time (P < 0.0001); however, the rate of pairs with one or both infants requiring oxygen or ventilation increased significantly through time (P < 0.0001). Fetal death risks declined for term twins. Conclusions: The proportion of twins born at 32-36 weeks' gestation has increased over time, along with requirement for respiratory support. Twin pair mortality decreased from 1980 to 2005.