In most western countries the newborn sex ratio (NSR) at birth (number of males for each female in a population) is around 105 boys to 100 girls while for example in China or Vietnam it is above 110. The objective of this study was to calculate the NSR in the United States by mother's and father's race and ethnicity.METHODS:
This is a retrospective cohort analysis of the 2013 CDC birth certificate data. The NSR was calculated for maternal and father's race and ethnic combinations including Hispanic, whites, blacks, and subgroups of Asians (A). We only included groups which had a minimum of 10,000 deliveries total.RESULTS:
There were 3,932,181 deliveries total. Asian mothers has a significantly higher NSR than non-Hispanic Black mothers (106.65 vs 103.14; P<.0001). Among Asian subgroups, Korean and Chinese mothers had the highest NSR when compared to Asian Indians (110.46 and 109.44 vs 104.43; P<.0001). The mother/father combination of Chinese mother/Asian father had the highest NSR (109.44) and the lowest NSR were to a combination of White mother/Black father (102.18) followed by Black mother/Black father (103.96). The range of NSR for Black fathers was 102.18–103.96 and for Asian fathers it was 104.41–110.09.CONCLUSION:
The surprising findings of this study are the significant differences of newborn sex ratios and the influence of the father among different races and ethnicities in the United States with Asians having higher NSR than Hispanics, Blacks or Whites. Further research is warranted to determine causes.