Trends in Birth Rates, Sex Ratio, and Infant Mortality Rates Among Singletons, Twins, and Higher Order Multiples in Brazil Between 2010 and 2023

This study examines birth rates and infant mortality rates in Brazil among singletons, twins, and higher order multiples between 2010 and 2023. Data were obtained from the Brazilian Ministry of Health's Live Birth Information System and Infant Mortality Information System. The dataset comprised 39,663,928 live births and 464,843 infant deaths occurring within the first year of life. Twin birth rates increased steadily over time, while singleton birth rates declined. The highest twin birth rates were observed in the Southeast (11.64‰) and South (11.47‰), whereas the lowest were observed in the North (7.81‰) and Northeast (9.23‰). Infant mortality among twins was approximately five times higher than among singleton, and higher order multiples faced an approximately threefold greater risk compared to singletons. Infant mortality rates were highest in the North and Northeast and lowest in the Southeast and South. Sex ratios varied by gestation type, with singletons showing the expected male bias and twins a slightly reduced male proportion, reflecting biological and regional influences on birth rates in Brazil. The data reveal a marked demographic shift between 2010 and 2023, with birth rates declining among women in their teens and 20s and increasing among those in their late 30s and 40s. Using Weinberg's differential method, we identified divergent trends in zygosity: dizygotic birth rates increased significantly over time, whereas monozygotic rates remained stable. This contrast underscores the environmentally responsive nature of dizygotic twinning, particularly its association with advancing maternal age and assisted reproductive technologies, compared to the relatively biologically stable pattern of monozygotic twinning.