Sex differences in birth weight depending on the mother's condition: testing the Trivers-Willard hypothesis in Indian twins

According to the Trivers-Willard hypothesis, mothers who give birth to sons when their general condition is good and daughters when their condition is bad have an advantage in fitness. The purpose of this study was to test this hypothesis in humans by examining sex differences in birth weight according to maternal age based on a comparison of the birth weights of twins. A total of 2138 Indian twins (668 opposite sex, 2940 same sex) from the 2015–2016 National Family Health Survey IV database were identified for analysis. In total, 3700 mothers were at low risk for having low-birth-weight babies in terms of maternal age (ages 20–34 years) and 576 mothers were at high risk in terms of maternal age (20 years younger or 35 years or older). The results of the analysis of covariance showed that: 1) the birth weight ratio of opposite-sex twins is small (female newborns are heavier) when the mothers are at a high risk age. 2) At a high risk age, female newborns in opposite-sex twins, who can receive sex-based discriminatory investments, are heavier at birth than female babies in same-sex twins. These results remained significant after controlling for variables potentially related to birth weight. This study provides potential evidence that in utero selection is retained by the mother as a countermeasure against threats in terms of fitness.