The effects of Thanksgiving, Christmas and Valentine's Day on the sex ratio at birth in the United States, 2003–2015

Abstract

IntroductionMales are born slightly in excess of females and many factors have been shown to influence M/F, the sex ratio at birth. Seasonality has also been shown to impinge on M/F. This study was carried out in order to ascertain whether Thanksgiving Day, Christmas Day and Valentine's Day had any influence on M/F in the United States population, nine months later. MethodsBirths by ethnicity, region and sex were obtained from the website of the Centres for Disease Control. Analyses were applied to seasonally adjusted time series and non-seasonally adjusted series. A seasonally adjusted indirect result was calculated for the sex ratio. ResultsThis paper studied 53,105,069 live births for 2003–2015 (27,178,968 males, 25,926,101 females). M/F rises exceeding 90th and 95th percentiles were strongest for the seasonally adjusted series for all births in regions: South (CENS-R3), West (CENS-R4) and the time series All-regions_All-race. When comparing unadjusted and seasonally adjusted series, a similar pattern was observed in the regions Northeast and Midwest for American Indian or Alaska Native and Asian or Pacific Islander. A similar pattern was observed for the region Midwest for the ethnic group White. DiscussionM/F rose above the 90th percentile in all the series and occasionally above the 95th percentile. Increased periconceptual coital rates increases M/F and this study thus lends further credence to the hypothesis that coital rates around the time of conception causally influences the sex ratio of subsequent births nine months later, possibly due to a hormonal mechanism.