Moderate exposure to cold and heat during early pregnancy may affect fetal development as early as the first trimester, according to a new study led by the Barcelona Institute for Global Health (ISGlobal), a centre supported by the “la Caixa” Foundation. The findings, based on two Dutch birth cohorts and published in the International Journal of Epidemiology , suggest that early gestational development may be sensitive to ambient temperature , with potential implications for birth outcomes and long-term health as climate conditions continue to change.
Previous studies have linked exposure to hot or cold temperatures during pregnancy to a higher risk of complications and adverse birth outcomes. However, whether ambient temperature influences development during the earliest stages of pregnancy remains unclear. The first trimester is a critical period, as the baby’s organs and the placenta begin to form; alterations during this stage have previously been associated with adverse birth outcomes and with cardiovascular and respiratory conditions later in childhood.
“To evaluate the association between ambient temperature exposure and embryonic and early fetal development, we analysed data from a Dutch birth cohort, the Generation R Next Study (2017–2021), and replicated the findings in an independent cohort established 15 years earlier, the Generation R Study (2002–2006),” explains Esmée Essers , researcher at ISGlobal and first author of the study. The researchers estimated weekly average temperatures from the beginning of pregnancy and used ultrasound scans at around 8, 10, and 12 weeks to measure crown–rump length , a standard indicator of embryonic and early fetal growth.
Exposure to both colder and warmer ambient temperatures during the first trimester was associated with a smaller crown–rump length at 12 weeks of gestation in the Generation R Next Study cohort. The association between colder temperatures and smaller crown–rump length at 12 weeks was also observed in the independent cohort, despite differences in temperature patterns between the study periods.
The timing of exposure appeared to differ between heat and cold. Exposure to higher temperatures showed stronger associations during early pregnancy, particularly between weeks 1 and 6 . In contrast, exposure to colder temperatures was associated with CRL across a longer period, from weeks 1 to 11 . In both cases, no associations were detected for earlier ultrasound measurements at 8 or 10 weeks.
At 12 weeks of gestation, exposure to both colder and warmer temperatures was associated with a crown–rump length around 7–8 millimetres smaller than expected for this stage of development. The researchers also observed differences between the two cohorts regarding when temperature exposure appeared to have the strongest associations and regarding the magnitude of the observed effects. “These differences could be related to changes in climate conditions over time, differences in how populations respond or adapt to temperature exposure, or other factors that may influence vulnerability during pregnancy”, explains Essers .
Physiological changes during pregnancy place additional demands on the body’s ability to regulate temperature. “External exposure to heat or cold may further challenge this balance, triggering thermoregulatory responses such as changes in peripheral blood flow,” explains Mònica Guxens , ICREA Research Professor at ISGlobal and coordinator of the study. “These responses could potentially influence uterine perfusion and, in turn, affect early developmental processes. However, further research is needed to understand the biological mechanisms involved and whether these early differences have implications later in pregnancy.”
Reference
Essers, E., de Groot, J. M., Graafland, N., Gonçalves, R., Iñiguez, C., Petricola, S., Jaddoe, V., El Marroun, H., Tiemeier, H., Steegers, E., Rousian, M., Mulders, A., & Guxens, M. (2026). Associations of ambient temperature exposure with embryonic and early fetal development. International Journal of Epidemiology, 55(3) . https://doi.org/10.1093/ije/dyag060
International Journal of Epidemiology
Observational study
People
None declared.