RESUMEN
CONTEXT: A previous study showed that significantly more boys were born in southern latitudes in Europe than in northern latitudes and the converse pattern was observed in North America. OBJECTIVE: This study analyses secular trends in gender ratios for live births over the second half of the 20th century. DESIGN, SETTING, PARTICIPANTS: Analysis was carried out from a World Health Organisation dataset comprising live births over the above period. This included 127034732 North American and 157947117 European live births. MAIN OUTCOME MEASURES: Analysis of trends in gender ratios for countries in both continents. RESULTS: The findings show a highly significant overall decline in male births in both Europe and North America (p<0.0001), particularly in Mexico (p<0.0001). Interestingly, in Europe, male births declined in North European countries (latitude>40 degrees, p<0.0001) while rising in Mediterranean countries (latitude congruent with 35-40 degrees, p<0.0001). These trends produced an overall European male live birth deficit 238693 and a North American deficit of 954714 (total male live birth deficit 1193407). CONCLUSIONS: No reasonable explanation/s for the observed trends have been identified and the causes for these trends may well be multifactorial.
Asunto(s)
Tasa de Natalidad/tendencias , Razón de Masculinidad , Europa (Continente)/epidemiología , Femenino , Humanos , Recién Nacido , Masculino , América del Norte/epidemiología , Distribución por SexoAsunto(s)
Razón de Masculinidad , Europa (Continente) , Femenino , Humanos , Masculino , América del NorteRESUMEN
OBJECTIVE: Demographic studies in various industrialised countries have shown a decline in male births in the latter half of the 20th century from the expected ratio of 0.515 (males/total). This study analyses trends in this ratio over the period 1890 to 1995 in Malta, and also analyses this ratio for Western European countries for the period 1990-1995. DESIGN: Births subdivided by sex were obtained from official Maltese publications. European countries were grouped according to geographical latitude by banding countries into three groups: Northern Mediterranean, Central European and Scandinavian. Births by sex for these countries were also analysed for the period 1990-1995. RESULTS: No decline in the ratio of male births to total births was noted in Malta over the period 1916-1995. However, the ratio was higher than expected (n = 151,766, ratio = 0.517 (95% confidence intervals (95% CI): 0.514, 0.519). Moreover, during the period 1890-1899 (n = 66,874), the ratio was 0.523 (95% CI: 0.519, 0.527), even higher than observed during the 20th century (chi 2 = 8.3, p = 0.004). Analysis of European births showed a much higher ratio of male births in the south of Europe than in the north (chi 2 = 87.2, p < 0.0001). CONCLUSIONS: The findings were unable to explain the higher incidence of male births in the south of Europe, but it is speculated that ambient temperatures may not only affect fertility, but also influence sex ratios at birth.