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J Perinatol. 2008 May;28(5):354-60. doi: 10.1038/ Epub 2008 Feb 14.

Infants born at the threshold of viability in relation to neonatal mortality: Colorado, 1991 to 2003.

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Department of Pediatrics, Section of Neonatology, University of Colorado at Denver and Health Sciences Center, Denver, CO 80045, USA.



To determine the contribution of infants born at the threshold of viability (defined as <750 g birth weight) and the role of regionalization of perinatal care on the neonatal mortality rate (NMR) in Colorado.


We performed a retrospective cohort study, evaluating all live births in Colorado from 1991 to 2003, and comparing the periods 1991 to 1996 versus 1997 to 2003.


The overall unadjusted NMR of the two time periods was 4.3 and 4.4 per 1000 live births, respectively (P=0.42). The contribution of infants with birth weights<750 g to the overall NMR increased from 45.0 to 54.5% (P<0.01). The odds of death for infants<750 g increased between time periods (Odd ratio 1.3, 95% Confidence interval 1.11, 1.61). However, NMR decreased between time periods for all birth weight categories, until infants<600 g. With respect to regionalization, the number of infants<750 g born in a level III care center increased slightly between the two time periods (69.6 versus 73.3%; P=0.04); however, adjusted analysis showed no difference in the practice of regionalization between time periods. Regardless of time period, infants who weighed <750 g born in a level III center had 60% lower mortality risk when compared to <750 g infants born in a non-level III center (P<0.01; 95% CI 0.30, 0.52).


Despite advances in neonatal medicine, the overall NMR in the state of Colorado remained unchanged between the time periods of 1991 to 1996 and 1997 to 2003. Infants at the threshold of viability continue to have a large impact on the Colorado NMR, making up a larger proportion of overall neonatal deaths. While the results demonstrate that the risk of mortality is significantly reduced for <750 g infants born in a level III center, the practice of regionalization has not changed between the two time periods. Improved efforts to standardize the referral practices to ensure delivery of <750 g infants in level III centers could potentially reduce the impact of these infants on the NMR. While the overall NMR in Colorado has not changed between the two time periods, the NMR for infants>600 g has significantly decreased, suggesting that the boundary delineating the threshold of viability needs reevaluation, as it may have been pushed lower than previously defined.

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