Increasing rates of sex-discordant twins no longer correspond to decreasing perinatal mortality rates.
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To analyze dizygotic twinning rates and outcomes over a 25-year period.Birth and fetal death certificates from 1980-2004 in Washington State, USA, were analyzed retrospectively to find factors associated with the increase in sex-discordant twins through time. "Low" and "high" fertility treatment groups were defined according to demographic traits. Perinatal mortality was defined as fetal or neonatal death of one or both twins and Weinberg's rule was used to estimate mortality for monozygotic and dizygotic pairs.Controlling simultaneously for maternal age, race, parity, and education did not eliminate the trend of increasing sex-discordant twins from 1992-2004 (M-H chi2 P=0.001). The "low" fertility group had a non-significant decline in sex-discordant twins (M-H chi2 P=0.24), whereas the "high" fertility group had a significant increase (M-H chi2 P=0.001). Perinatal mortality decreased for monozygtic twin pairs throughout the study period, but decreased until the mid-1990s and then increased slightly through 2004 for the dizygotic twin pairs.Advancing maternal age and increasing use of fertility treatments are largely responsible for the increase in dizygotic twins from 1980-2004 and may also be responsible for the stalling of the decline in perinatal mortality rate.