Changes in infant mortality among extremely preterm infants: US vital statistics data 1990 vs 2000 vs 2010

Infant mortality among extremely preterm infants (22 to 28 weeks gestation) varies considerably by gestational age. The reduction in mortality over a 20-year period, when examined in gestational age week increments, may give a more precise estimate of progress or lack thereof in caring for these infants and provide information to better inform practitioners and parents of the risk of mortality among these small infants. The objective of this analysis is to examine infant mortality (birth to 365 days) by week of gestation for infants 22 to 28 weeks gestation comparing mortality rates, adjusting for maternal and infant birth characteristics, among US births for the years 1990, 2000 and 2010.

US vital statistics period-linked birth and infant death certificate files for the years 1990, 2000 and 2010 were used. Maternal and infant characteristics for births at 22 to 28 weeks were abstracted from the files. A trimming procedure was used to remove records that had birth weights that exceeded the interquartile range of birth weights for a given week of gestational age. Infant mortality rates were calculated, and adjusted odds ratios for mortality were generated using logistic regression models.

A total of 15,593 live births, 22 to 28 weeks gestation were available for the year 1990; 17,095 for the year 2000; and 14,721 for the year 2010. Infant mortality rates ranged from 904 per 1000 live births at 22 weeks gestation in 1990, to 835 in 2000, to 866 in 2010. Across all gestational age groups there was an adjusted reduction in the odds ratio for mortality of ~50% from 1990 to the year 2000. However, between 2000 and 2010 there was no significant reduction in infant mortality except at 25 weeks gestation (adjusted odds ratio=0.81, 95% confidence interval=0.70, 0.93).

Despite a significant reduction in infant mortality among extremely preterm infants between the years 1990 and 2000, there has been little progress in reducing mortality between the years 2000 and 2010.