Male-to-Female Ratios, Race/Ethnicity, and Spontaneous Preterm Birth among 11 Million California Infants

Objective An observed disparity in population-scale data are a larger number of males among preterm births (PTBs). We investigated spontaneous PTB risk among women of various race/ethnic groups in combination with infants' sex. Study Design This observational study was conducted in>10 million California births (1991-2012) using birth certificates linked with maternal and infant hospital discharge data. Results Male-to-female ratios among term (37-42 weeks) infants exhibited the narrow ratio range 1.02 to 1.06 across race/ethnic groups. Such ratios among spontaneous PTBs were generally larger for all race/ethnic groups except non-Hispanic blacks. For blacks, ratios tended to be lower and similar to their term birth counterpart, 1.03. Hazard ratios adjusted for maternal age and education for non-Hispanic blacks were 0.99 (95% confidence interval [CI] 0.90-1.09), 1.01 (95% CI 0.95-1.08), 0.98 (95% CI 0.94-1.03), and 1.03 (95% CI 1.01-1.05), respectively, for gestational week groupings of 20 to 23, 24 to 27, 28 to 321, and 32 to 36. Hazard ratios for non-Hispanic whites for the same groupings were 1.08 (95% CI 0.98-1.18), 1.13 (95% CI 1.07-1.19), 1.21 (95% CI 1.17-1.25), and 1.18 (95% CI 1.17-1.19). Conclusion Why male-to-female ratios are similar across gestational ages in blacks but substantially higher in other race/ethnic groups is theoretically considered relative to inflammation, stress, and other influences.

Automated summary

An analysis of over 10 million California births from 1991 to 2012 revealed that disparities in spontaneous preterm birth risk among women of different race/ethnic groups were influenced by the sex of the infants. Male-to-female ratios in preterm births were generally higher for all groups except non-Hispanic blacks, where the ratios remained similar to term births. Possible explanations for these differences include factors such as inflammation, stress, and other environmental influences.