Our study is based on a large population registry data, which produces statistically stable results and allows an examination of the effects of parental race on different components of fetal and infant death. To our knowledge, this is the first study to explore the association between the mixed race and the fetal and infant mortality in twins.
Our study found that the fetal and infant mortality rates in the B-B group and infant mortality rate in the FB-MW group were significantly higher than that of in the W-W group. buy generic viagra
Adjustment for parental age, maternal education, cigarette smoking during pregnancy, prenatal care service, and FGR did not change the results. After preterm birth and VLBW were added to the risk-free analysis, however, the risk of fetal death in the B-B group became not higher than the W-W group. The higher infant mortality rates in the B-B and FB-MW groups remained even after adjustment for preterm birth and VLBW. These results indicate that both maternal and paternal race are associated with the risk of fetal and infant mortality, with higher impact of paternal race on the infant mortality than maternal race, and that the higher risk of fetal mortality in twin infants born to black parents are largely attributed to their higher incidence of preterm deliveries (secondary probably to their higher exposure to environmental risk factors of preterm births such as infection, stress, cigarette smoking, etc., not necessarily the genetic factors). These findings can be useful for physicians and public health workers caring for population with large ethnic minorities, and for future researches investigating the racial/ethnic differences in health.
The FB-MW group is unique in that despite their much higher maternal smoking rate and higher proportions of teenage mothers and low maternal education than both the B-B and FW-MB groups, the preterm birth and VLBW rates in them were lower than both B-B and FW-MB groups. Reasons for such a paradox deserve further investigation by primary data collection studies in the future.
In a study assessing the magnitude of the disparity in infant mortality between twins born to black and white teenagers based on twins born to adolescents in the United States in the period 1995-1997, Salihu et al. found that infant mortality was 20% higher among black twins as compared to their white counterparts, and that the black-white disparity in infant mortality occurred exclusively in the neonatal period. They conclude that the higher proportion of small-for-ges-tational age instead of preterm births among black twins was the most likely explanation for the higher infant mortality among twins born to black teenagers. Although both studies by us and by Salihu et al. used the same database, differences in study design (e.g., the study by Salihu et al. included only teenagers and examined only the effects of maternal race whereas our study included all black and white subjects and examined the effects of both parents’ race) and analysis (the study by Salihu et al. used generalized estimating equations whereas our study used “risk-free” analysis) make it difficult to reconcile the results of the two studies.
The higher postneonatal mortality rate in infants born to black parents may have explanations beyond pregnancy. Cheung et al. found that birth size was strongly associated with neonatal mortality but only weakly associated with the postneonatal mortality. Perhaps postneonatal death is more related to infant environment than pregnancy experience.
Alexander et al. reported that the neonatal mortality rates were 3.24 and 8.16 per 1,000 live births, respectively for whites and blacks in the United States. MacDorman et al. found that the rates of neonatal death and postneonatal death and fetal death ratio were 2.47, 2.47, and 2.21 times, respectively, in blacks as compared with whites. The risk ratio of fetal and infant mortality between blacks and whites found in these previous studies that included all births (majority were singletons) were higher than that of in twins observed in our study. In a comparison of infant mortality between infants born to black and white mothers, Kleinman et al. also found that the black-white risk ratio in infant mortality was larger in singletons than in twins. This has happened because the LBW rate is higher in twins and fetal and infant mortality rates in LBW black infants are actually lower than LBW white infants.
There is a current debate on whether birthweight should be adjusted for in the comparison of infant mortality among different populations. We believe that both results should be presented, because they convey information with different implications. For example, if the difference in fetal and infant mortality between two populations (e.g., blacks and whites) can be totally explained by differences in LBW rate, research and public health efforts aiming at improving infant health in the disadvantaged population (e.g., blacks) should be focused on reducing LBW in them. Otherwise other measures (e. g., improving access to quality perinatal care services) should be implemented. LBW is caused by a slow fetal growth or a short stay in uterus (preterm birth), or a combination of the two. Comparison of results adjusting for fetal growth or gestation duration separately could yield more meaningful information. For example, in our study, adjustment for FGR did not change the results. However, after adjusting for preterm birth or LBW, the effect of parents’ race on fetal mortality disappeared. These results suggest that much of the observed black-white (or mixed race) differences in fetal mortality could be explained by the higher preterm birth rate in blacks or mixed race groups, not by differences in fetal growth restriction.
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