Cumulative hazard of stillbirth at a gestation interval estimates the probability of having a stillbirth while in that given interval of pregnancy. The cumulative hazard for stillbirth /1000 pregnancies from 20 through 41 weeks gestation was 22.07 for non-Hispanic blacks, 10.02 for non-Hispanic whites, and 10.58 for Hispanics. The stillbirth hazard was highest at 20-23 weeks and 39-41 weeks gestation (). Increased hazard was observed at every gestation interval for blacks compared to the other racial/ethnic groups. The black/white disparity in cumulative hazard was highest at 20-23 weeks, (RR 2.75 [95% C. I. 2.62, 2.88]), and declined with increasing gestation, reaching the lowest value at 39-40 weeks, (RR 1.57, [95% C.I. 1.41, 1.75]) () then rising slightly at 41 weeks of gestation. The hazard of stillbirth for Hispanics was similar to non-Hispanic whites throughout pregnancy.
Hazard of stillbirth for singleton pregnancies by gestational age and race/ethnicity, 2001-2002
Cumulative Hazard of Stillbirth At Intervals of Gestation According to Maternal Race/ethnicity
The distributions of reported maternal demographic factors and maternal and fetal conditions among live births and stillbirths are listed in . Among all race/ethnicities, a higher proportion of stillbirths were to women 35 or older, or nulliparous women than the proportion of live births. In addition, maternal medical, pregnancy, and labor conditions were more frequently reported on fetal death certificates than birth certificates. A high proportion of stillborn fetuses among all race/ethnicities were small for gestational age: 25 to 29% of stillbirths, compared to 5 % of live births. The proportion of stillbirths with any reported congenital anomaly was higher than the proportion of live births with anomalies, with the highest proportion occurring among whites. The percent of missing records varied by race/ethnicity and outcome. For example, they ranged from 0.21 to 2.90 for parity, 0.82 to 11.36 percent for education, and 4.34 to 19.87 for any maternal condition (medical, pregnancy, or labor).
Distribution of Reported Maternal and Fetal Characteristics
The cumulative hazard of stillbirth through 41 weeks was examined according to race/ethnicity and maternal characteristics. The influence of education varied by race/ethnicity, with higher education benefiting white and Hispanic women more than black women (). The stillbirth hazard for white women with >12 years of education was 30 percent lower than the hazard for white women with ≤12 years of education (RR= 0.70 [95% C.I. 0.67,0.72]). The influence of higher educational level on reducing cumulative hazard of stillbirth was much less for blacks (RR=0.91 [95% C.I. 0.86,0.95]) and not significant for Hispanics (RR=0.96 [95% C.I. 0.90, 1.02]). This led to an increased black/white disparity in stillbirth hazard among women with more than 12 years of education compared to women with ≤12 years of education, with the disparity declining with advancing gestation (). The black/white disparity in hazard for women with >12 years of education was highest at 20-27 weeks, RR=2.98 (95% C.I. 2.79, 3.18). The Hispanic/white disparity in cumulative hazard was also significantly higher for those with more than 12 years of education, RR=1.22 (1.15, 1.30, 95% C.I.) when compared to those with 12 years or less, RR=0.89 (0.86, 0.92, 95% C.I.).
Cumulative Hazard of Stillbirth Between 20 and 41 weeks According to Maternal Characteristics
Disparity in Risk At Intervals in Gestation According to Maternal Characteristics
The hazard for women ≥35 years old was higher than for women under 35 (). The relative risk (RR) of stillbirth hazard for advanced maternal age was highest for Hispanic women (RR=1.66 [95% C.I. 1.55, 1.78]), followed by black women (RR=1.51 [95% C.I. 1.41, 1.61]), and white women, (RR=1.38 [95% C.I. 1.33, 1.44]). The magnitude of the black/white and Hispanic/white disparities in stillbirth hazard for women ≥ 35 years of age was greatest at 28-36 weeks of gestation ().
The relative risk for multiparous women was 60% or more lower than nulliparous women (). There was a 20-30% increased disparity in stillbirth risk for Hispanic multiparous women compared to white multiparous women, which was not observed among nulliparous women (.). The black/white disparity in risk was similarly increased for multiparous and nulliparous women at each gestational interval.
To examine the contribution of maternal and fetal conditions to the hazard of stillbirth across gestation, we determined the hazard in the population of women without the condition. (). The contribution of reported maternal medical conditions to cumulative hazard through 41 weeks ranged from 2.56-5.35%. The contribution of medical conditions increased with gestational age. The contribution of any pregnancy condition (incompetent cervix, premature rupture of membranes, uterine bleeding, pregnancy associated hypertension or eclampsia) to hazard of stillbirth was greater: 7.96, 14.92, and 9.63 percent for whites, blacks, and Hispanics respectively. The greatest contribution of pregnancy conditions to stillbirth hazard occurred at less than 27 weeks of gestation. The contribution of reported labor related conditions (febrile, abruption, cord abnormalities, placenta previa or other bleeding) to cumulative hazard of stillbirth was 13.67, 17.49, and 10.87 percent for whites, blacks, and Hispanics, respectively. The contribution of any maternal condition (reported medical, pregnancy, and labor conditions combined) to stillbirth hazard was 19.56% for non-Hispanic whites, 19.28% for Hispanics and 30.09% for blacks.
Cumulative Hazard of Stillbirth at Intervals of Gestation in the Absence of Specified Maternal or Fetal Conditions
Small for gestational age (SGA) fetuses, defined as below the 5% percentile of birth weight for race/gender/parity matched liveborns in the same gestational age, accounted for 20 to 25% of the stillbirth hazard. The contribution of any reported congenital anomalies to stillbirth hazard decreased as gestation progressed. The contribution of congenital anomalies to stillbirth hazard was 19.66% for whites, 10.60% for blacks, and 15.69% for Hispanics. The contribution of SGA and congenital anomalies to stillbirth hazard was greater for whites compared to blacks at preterm gestations, especially between 20-27 weeks, but equal at 37-41 weeks.