A 2009 research paper published by Branum et al. reports the sex ratio derived from data in United States birth records over a 25-year period (1981–2006).[28] This paper reports that the sex ratio at birth for the white ethnic group in the United States was 1.04 when the gestational age was 33–36 weeks, but 1.15 for gestational ages of less than 28 weeks, 28–32 weeks, and 37 or more weeks. This study also found that the sex ratios at birth in the United States, between 1981 and 2006, were lower in both black and Hispanic ethnic groups when compared with non-hispanic white ethnic group.