Racial and ethnic disparity in male preterm singleton birth

To evaluate the association between male sex and increased preterm birth (PTB) by race and ethnicity among singleton fetuses born in the United States.

The 2002 National Center for Health Statistics Natality database was analyzed for known risk factors of PTB by fetal sex and race. Multivariable Cox proportional hazards regression was applied to estimate the hazard ratio by fetal sex for births between 20 and 36 completed gestational weeks. The multivariable model was adjusted for known demographic, medical and obstetric risk factors for PTB. Subgroup analysis was performed using both race and ethnicity.

In the overall analysis 3,853,678 singleton live births were included. PTBs between 20 and 36 completed weeks occurred in 10.9% of male and 9.8% of female infants. The association between PTB and male sex persisted after adjustment for other risk factors and did not vary by gestational age (hazard ratio [HR] 1.13, 95% CI 1.12-1.14). This association was seen in all racial and ethnic groups. This effect was less prominent among black infants (HR 1.06, 95% CI 1.04-1.07). Male fetuses with Hispanic ethnicity had a significantly higher risk of preterm birth (HR 1.15, 95% CI 1.13-1.17) compared to male fetuses of non-Hispanic ethnicity (HR 1.12, 95% CI 1.12-1.13).

In the U.S. male fetuses are at higher risk for PTB compared with female fetuses at the same gestational age. The etiology of this difference is unclear, but the effect is reduced for black compared to white fetuses and increased for Hispanic fetuses.