Changes in stillbirth and infant mortality associated with increases in preterm birth among twins

Objective. To examine whether the recent substantial increase in preterm birth among twins has been associated with changes in fetal and infant mortality. Design. Cohort study based on information in the linked live birth, stillbirth, and mortality databases of Statistics Canada. Setting. Ten of 12 provinces and territories in Canada. Participants. All twin live births and stillbirths between 1985 and 1996, along with information on deaths during infancy (1985-1997). Main Outcome Measure. Fetal and infant mortality rates. Results. The rate of preterm birth among twin live births increased significantly by 17% (95% confidence interval: 14%-20%) from 42.5% between 1985 and 1987 to 49.6% between 1994 and 1996. Overall, stillbirth rates among twins declined from 22.4 per 1000 total births in 1985 to 1987 to 18.8 per 1000 total births in 1994 to 1996. Among twin fetuses greater than or equal to 34 weeks' gestation, stillbirth rates decreased from 9.5 per 1000 in 1985 to 1987 to 5.4 per 1000 fetuses at risk in 1994 to 1996. Infant mortality rates among twin live births declined substantially in all categories of gestational age above 24 weeks except for live births at 32 to 33 and 34 to 36 weeks' gestation. Conclusions. The recent increase in preterm birth among twins was associated with a substantial reduction in stillbirth rates at and near term gestation. Infant mortality rates declined concurrently, although the absence of a significant decrease in infant mortality among twin live births at 32 to 33 and 34 to 36 weeks' gestational age needs additional scrutiny.