Objectives.

Declines in neonatal mortality have been attributed to neonatal intensive care. An alternative to the “better care” hypothesis is the “better babies” hypothesis; ie, very low birth weight infants are delivered less ill and therefore have better survival.

Design.

We ascertained outcomes of all live births <1500 g in two prospective inception cohorts. We estimated mortality risk from birth weight and illness severity on admission and measured therapeutic intensity. We calculated logistic regression models to estimate the changing odds of mortality between cohorts.

Patients and Setting.

Two cohorts in the same two hospitals, 5 years apart (1989–1990 and 1994–1995) (totaln = 739).

Results.

Neonatal intensive care unit mortality declined from 17.1% to 9.5%, and total mortality declined from 31.6% to 18.4%. Cohort 2 had lower risk (higher birth weight, gestational age, and Apgar scores and lower admission illness severity for newborns ≥750 g). Risk-adjusted mortality declined (odds ratio, 0.52; confidence interval, 0.29–0.96). One third of the decline was attributable to “better babies” and two thirds to “better care.” Use of surfactant, mechanical ventilation, and pressors became more aggressive, but decreases in monitoring, procedures, and transfusions resulted in little change in therapeutic intensity.

Conclusions.

Mortality decreased nearly 50% for infants <1500 g in 5 years. One third of this decline is attributable to improved condition on admission that reflects improving obstetric and delivery room care. Two thirds of the decline is attributable to more effective newborn intensive care, which was associated with greater aggressiveness of respiratory and cardiovascular treatments. Attribution of improved birth weight specific mortality solely to neonatal intensive care may underestimate the contribution of high-risk obstetric care in providing “better babies.”

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