Socioeconomic determinants of extreme perinatal outcomes in Spain, 1996-2022.

Adversities during foetal development and early life are important determinants of health across the life course, as exposures during sensitive developmental stages may lead to negative outcomes later in life. Mortality represents the most extreme consequence of such adversities. In this study, we focus on perinatal mortality, defined as registered deaths occurring from the 22nd week of gestation up to the first 24 hours of life. Understanding the factors that increase the likelihood of extreme events during this critical developmental stage is essential for prevention. We assess the impact of socioeconomic factors-maternal origin, educational attainment, and municipality size of residence-on perinatal mortality, controlling for maternal age. Joint conditional statistical analyses are based on a mixed-effects Poisson regression model for 2007-2022, with random effects accounting for unobserved heterogeneity across calendar years. Among the factors analysed, origin and education emerge as the most relevant. Results indicate that foreign-born mothers exhibit substantially higher perinatal mortality risks, with estimated IRRs ranging from 2.2 (CI: 2.1-2.4) to 6.5 (CI: 5.7-7.2), depending on the mother's continent of origin. Higher maternal education is associated with a reduced risk of perinatal mortality, with a mean relative risk (IRR) of 0.64 (CI: 0.60-0.68) compared to mothers with primary education or less. Maternal age also plays a key role: teenage mothers (≤ 20 years) and mothers of advanced age (≥ 37 years) experience markedly higher perinatal mortality. Overall, this study identifies maternal profiles most at risk and quantifies disparities in perinatal health across socioeconomic characteristics in a country with universal, free healthcare.