Exposure to elevated temperatures during the nine months before conception significantly alters the proportion of male versus female births, with heat stress reducing male birth rates. This finding emerges from demographic analysis spanning multiple populations and suggests that thermal stress may influence early embryonic development or sperm viability in ways that favor female offspring. The temperature-sex ratio connection adds a new dimension to understanding how climate change might affect human reproduction beyond fertility rates alone. Previous research has established that various stressors can skew sex ratios, but temperature's role remained unclear despite its fundamental influence on biological processes. The mechanism likely involves either heat-induced damage to Y-chromosome-bearing sperm, which are more vulnerable than X-chromosome carriers, or differential embryonic survival rates under thermal stress. For couples trying to conceive, this suggests that seasonal timing and heat exposure during the periconceptional period may have unintended demographic consequences. However, the effect sizes appear modest, and individual reproductive outcomes remain largely unpredictable. As global temperatures rise, this phenomenon could contribute to subtle but measurable shifts in population sex ratios, though other environmental and social factors likely exert stronger influences on reproductive patterns.