Sex differences in fetal mortality in Spain: historical trends and social and biological determinants

OBJECTIVE: To analyse historical sex differences in fetal mortality in Spain since 1900 and their association with biological, social, and contextual determinants between 1996 and 2019. METHOD: We conducted a population-based, retrospective observational study using official data from the Spanish National Statistics Institute. Fetal mortality rates and sex ratios were calculated and stratified by maternal age, gestational age, marital status, municipality size, maternal geographic origin, and ecological proxies of solar exposure. The analysis was strictly descriptive and ecological, without attempting causal inference. RESULTS: A persistent excess of male fetal mortality was observed throughout the study period. In the first decades of the twentieth century, male rates exceeded female rates by approximately 30-40%. As fetal mortality declined, the sex differential progressively narrowed, reaching around 5-10% in recent decades. Sex inequality varied non-linearly across gestation and differed by maternal age, residential environment, and geographic origin. The sex ratio was lower for conceptions occurring in months with higher solar irradiation and in southern peninsular provinces and the Canary Islands. CONCLUSIONS: Sex inequality in fetal mortality in Spain is persistent yet modifiable, and is consistent with the interaction between male biological vulnerability and social and contextual determinants. Incorporating fetal sex is essential for understanding perinatal inequalities.