The population distribution of 168 MSAs with sufficient numbers of black and white births is detailed by race, region, and metropolitan size in . The Southeast region had the largest number of MSAs, with the remainder roughly evenly divided among the other three regions. Segregation by either index tended to be lower in the West, as did black poverty rates. Smaller MSAs also had slightly lower levels of segregation, while the largest MSAs had lower black poverty rates.
Population distribution by metropolitan characteristicsa
The displays the distribution of VPT birth rates by race across all eligible MSAs for each racial/ethnic group. The x-axis is the rate of VPT birth per 1,000 live births, and the y-axis is the relative frequency of MSAs; in other words, it is the proportion of all MSAs at a given rate. For white women, the mean MSA rate of VPT birth was 12.3/1,000 (SD=2.7); for black women, the mean rate was 34.8/1,000 (SD=6.9); and for Hispanic women, the mean MSA rate was 15.7/1,000 (SD=4.0). Restricting the analysis to the 168 MSAs, which had sufficient white and black births, changed these estimates very little, as seen in . F-tests comparing the variance in white and black distributions of MSA rates were statistically significant (p<0.0001) for every stratum investigated. Similarly, the random selection of white births to equalize sample sizes only reduced the white SD by 7%, from 2.7 to 2.5, leaving the ratio of the variance in the black distribution to the white distribution relatively unchanged.
Rates of very preterm birth in U.S. metropolitan statistical areas (MSAs) by race, 2002–2004
Distribution of rates of very preterm births aggregated at the MSA level by race, 2002–2004
Within each racial group, the lowest mean rate was in Western cities (9.9, 26.2, and 13.6 per 1,000 live births for white, black, and Hispanic women, respectively), among married mothers (10.3, 29.4, and 12.7), and college-graduated mothers (7.9, 29.4, and 10.9). The respective high rates for each group varied, with white mothers seeing the highest mean rate among unmarried women (18.7) and women without a high school degree (19.0). Black women also had a high risk of VPT birth among unmarried mothers (37.7), as well as mothers residing in cities with the highest quintile of proportion black (39.2) and highest quintile of black women below the poverty line (37.5). For Hispanic women, high rates occurred among unmarried women (18.7), cities in the Northeast region (18.7), for nulliparous women (17.1) and women residing in cities with the highest quintile black population below poverty line (18.0). Cities with the highest unevenness segregation had lower VPT birth rates for white women (11.6 vs. 13.1 in the lowest quintile), while the opposite was true for black women (first quintile = 33.9, fifth quintile = 35.5). The effect of higher isolation segregation in black women was even stronger, with a range of 30.8 to 37.1 from the first to fifth quintiles.
Given equalization of the sample size, the SD of the distribution of city rates in each stratum could be a rough indicator of the similarity or variation among cities. For all three groups, the greatest city-to-city variation was seen when restricting to unmarried mothers (SD=3.4, 7.8, and 4.5 for white, black, and Hispanic women, respectively). Black women also had greater city-to-city variation in the group of cities with the lowest unevenness segregation (SD=9.4) and the highest income inequality (SD=8.9), while for white women, high variation was seen when restricting to mothers without a high school diploma (SD=4.1). For all three groups, city-to-city variation was lowest when restricting to mothers with a college education, and in the cities with the largest populations.
In addition to describing the within-race patterns of VPT birth rates, the means and SDs suggest varying interracial patterns. Black women consistently had two to three times the city-to-city variation for any given analysis as compared with white and Hispanic women. Although increasing education and marital status were protective for all groups, the magnitude of protection varied so that the black-white rate ratio among college-graduated mothers was 3.7 as compared with a ratio of 1.9 among women without a high school degree. The mean rate for U.S.-born Hispanic mothers was greater than the overall rate, suggesting increased risk with subsequent generations (mean of 15.7 overall, 17.2 for U.S.-born mothers). Hispanic women without a high school degree had lower rates than similarly schooled white or black women (16.2 vs. 19.0), although this advantage waned with increasing education.
The overall pattern of a threefold black-white racial disparity in mean rates, as well as a threefold increased variance, persisted with the use of VLBW as an alternate outcome. There was moderate correlation within cities of white and black VPT birth rates (r2 = 0.21, p<0.01) and of white and Hispanic rates (r2 = 0.20, p<0.01). Related is the observation that MSA-specific black/white rate ratios varied across MSAs with a median ratio of 2.8, and a range from 1.5 to 5.8.
Metropolitan area size and region of the country explained 26% of the black inter-city variance in VPT birth rates (). The black poverty rate and black median household income did not provide any better model fit than overall median household income alone, and so were dropped. The Gini coefficient was not significant in base models including region and metropolitan size, although both segregation indices were independently predictive in all models tested. A one SD increase in isolation segregation was associated with an increase in the black VPT birth rate of 2.7/1,000, while a one SD increase in unevenness reduced VPT rates to 2/1,000.
Linear regression of metropolitan characteristics and black very preterm birth rates per 1,000 live births