Temperature and sex ratios at birth

Area of Science:

• Environmental Health
• Reproductive Epidemiology
• Demography

Background:

• Human sex ratios at birth (SRBs) influence population dynamics and reflect maternal health and societal gender biases.
• Prenatal exposure to extreme heat is a potential environmental stressor that may affect SRBs, but empirical evidence is scarce.

Purpose of the Study:

• To investigate the association between prenatal heat exposure and sex ratios at birth across sub-Saharan Africa and India.
• To differentiate between spontaneous and sex-selective abortions influenced by temperature using sociodemographic factors and regional variations in son preference.

Main Methods:

• Analysis of survey data from 5 million live births in 33 sub-Saharan African countries and India.
• Linking birth data with high-resolution temperature data to assess prenatal heat exposure.
• Utilizing sociodemographic differentials, exposure timing, and regional son preference to distinguish abortion types.

Main Results:

• Maximum temperatures above 20°C were negatively associated with male births in both regions.
• In sub-Saharan Africa, first-trimester heat exposure correlated with fewer male births, suggesting increased spontaneous abortions, particularly among vulnerable maternal groups.
• In India, second-trimester heat exposure was linked to fewer male births, consistent with reduced sex-selective abortions, especially in higher-order births and among mothers in regions with strong son preference.

Conclusions:

• Prenatal heat exposure adversely affects maternal health and increases prenatal mortality, influencing family planning behaviors.
• Temperature-driven changes in spontaneous and sex-selective abortions lead to complex alterations in human sex ratios at birth.
• Environmental heat is a significant factor impacting reproductive outcomes and population sex composition.