Extremely preterm birth of a live newborn before the limit of viability is rare but contributes uniformly to the infant mortality rate because essentially all cases result in neonatal death.OBJECTIVE:
The objective of the study was to quantify racial differences in previable birth and their contribution to infant mortality and to estimate the relative influence of factors associated with live birth occurring before the threshold of viability.STUDY DESIGN:
This was a population-based retrospective cohort of all live births in Ohio over a 7 year period, 2006–2012. Demographic, pregnancy, and delivery characteristics of previable live births at 16 0/7 to 22 6/7 weeks of gestation were compared with a referent group of live births at 37 0/7 to 42 6/7 weeks. Rates of birth at each week of gestation were compared between black and white mothers, and relative risk ratios were calculated. Logistic regression estimated the relative risk of factors associated with previable birth, with adjustment for concomitant risk factors.RESULTS:
Of 1,034,552 live births in Ohio during the study period, 2607 (0.25% of all live births) occurred during the previable period of 16–22 weeks. There is a significant racial disparity in the rate and relative risk of previable birth, with a 3- to 6-fold relative risk increase in black mothers at each week of previable gestational age. The incidence of previable birth for white mothers was 1.8 per 1000 and for black mothers, 6.9 per 1000. Factors most strongly associated with previable birth, presented as adjusted relative risk ratio (95% confidence interval [CI]), were maternal characteristics of black race adjusted relative risk 2.9 (95% CI, 2.6–3.2), age ≥ 35 years 1.3 (95% CI, 1.1–1.6), and unmarried 2.1 (95% CI, 1.8–2.3); fetal characteristics including congenital anomaly, 5.4 (95% CI, 3.4–8.1) and genetic disorder, 5.1 (95% CI, 2.5–10.1); and pregnancy characteristics including prior preterm birth 4.4 (95% CI, 3.7–5.2) and multifetal gestation, twin, 16.9 (95% CI, 14.4–19.8) or triplet, 65.4 (95% CI, 32.9–130.2). The majority, 80%, of previable births (16–22 weeks) were spontaneous in nature, compared with 73% in early preterm births (23–33 weeks), 72% in late preterm births (34–36 weeks), and 65% of term births (37–42 weeks) (P < .001). Previable births constituted approximately 28% of total infant mortalities in white newborns and 45% of infant mortalities in black infants in Ohio during the study period.CONCLUSION:
There is a significant racial disparity in previable preterm births, with black mothers incurring a 3- to 6-fold increased relative risk compared with white mothers, most of which are spontaneous in nature. This may explain much of the racial disparity in infant mortality because all live-born previable preterm births result in death. Focused efforts on the prevention of spontaneous previable preterm birth may help to reduce the racial disparity in infant mortality.