During the first trimester, when a baby's organs and placenta are just beginning to form, even moderate shifts in temperature appear to leave a mark on fetal development — one that can be measured by ultrasound as early as 12 weeks of pregnancy. A new study from the Barcelona Institute for Global Health has found that exposure to both heat and cold during these earliest weeks of gestation is associated with noticeably smaller fetal size, raising important questions about how climate conditions may shape human development before most people even know they're pregnant.
The research matters because the first trimester is among the most vulnerable periods in human development. Scientists have long known that complications during these early weeks can cascade into adverse birth outcomes and even cardiovascular and respiratory conditions later in childhood. Yet until now, it remained unclear whether everyday temperature fluctuations—the kind most people experience without conscious effort—could influence this delicate stage of growth.
Researchers led by Esmée Essers, a scientist at ISGlobal, analyzed data from two independent Dutch birth cohorts spanning decades: the Generation R Next Study (2017–2021) and the earlier Generation R Study (2002–2006). They tracked weekly average temperatures from the moment of conception and used ultrasound scans at 8, 10, and 12 weeks to measure crown–rump length, the standard measure of embryonic and early fetal growth. Their findings, published in the International Journal of Epidemiology, revealed a consistent pattern: exposure to both colder and warmer temperatures during the first trimester was linked to a crown–rump length approximately 7–8 millimeters smaller than expected at 12 weeks of gestation.
What proved particularly striking was how the timing of temperature exposure differed between heat and cold. Exposure to higher temperatures showed stronger associations during the earliest weeks of pregnancy, particularly between weeks 1 and 6. Colder temperatures, by contrast, appeared to affect fetal development across a much longer window—from weeks 1 through 11 of gestation. Interestingly, no associations appeared in the earlier ultrasound measurements at 8 or 10 weeks, suggesting the effects crystallize by the third measurement point.
The mechanism remains incompletely understood, though researchers have a promising hypothesis. Pregnancy already demands significant effort from the body's temperature-regulation systems. "External exposure to heat or cold may further challenge this balance, triggering thermoregulatory responses such as changes in peripheral blood flow," explains Mònica Guxens, an ICREA research professor at ISGlobal who coordinated the study. These responses could potentially alter blood flow to the uterus, indirectly affecting early developmental processes. Yet Guxens notes that deeper biological research is needed.
The differences observed between the two cohorts—which studied populations 15 years apart during different climate periods—hint at complexity. Temperature exposure patterns themselves have shifted over time, and populations may adapt differently to thermal stress, or other unknown factors may influence vulnerability during pregnancy. These nuances underscore that this is not a simple story of temperature causing a fixed effect.
As climate conditions continue to shift globally, understanding how ambient temperature influences the earliest stages of human development takes on new urgency. The study suggests that pregnancy itself may represent a previously underappreciated window of sensitivity to environmental thermal stress—one that could have lifelong consequences.