11 Outcomes of Prenatal Heat Exposure

Overview

Prolonged or intense exposure to heat during pregnancy has been linked to a wide range of adverse birth outcomes, including miscarriage, stillbirth, preterm birth, growth restriction, and long-term developmental effects. These risks are highest when exposure coincides with critical windows of fetal development, especially during the second and third trimesters.

Miscarriage and Stillbirth

Early pregnancy is highly sensitive to environmental insults, including hyperthermia.

• Hyperthermia during the first trimester is associated with increased risk of spontaneous miscarriage and fetal loss (Edwards, 1995).

• Animal models show heat-induced placental damage, reduced uterine perfusion, and heightened inflammatory responses leading to fetal demise (Shiota & Opitz, 1982).

• In human epidemiological studies, extreme heat events correlate with a measurable increase in stillbirth risk, particularly late in pregnancy.

A study from California found that exposure to extreme heat in the week before delivery increased stillbirth risk by up to 10% (Basu et al., 2010).

Preterm Birth

Preterm birth—delivery before 37 weeks—is one of the most consistently associated outcomes of heat exposure.

• High maternal temperature activates the hypothalamic-pituitary-adrenal (HPA) axis, leading to increased cortisol and early labor initiation.

• Heat stress triggers prostaglandin release (especially PGE2 and PGF2α), which promotes cervical ripening and uterine contractions.

• Dehydration and reduced uterine blood flow under heat stress may also play a role.

A multi-country meta-analysis found that each 1°C increase in weekly average temperature increased the risk of preterm birth by 5% to 16% (Chersich et al., 2020).

Low Birth Weight and Growth Restriction

Fetal growth restriction (FGR) and low birth weight (LBW) are linked to maternal heat exposure through:

• Placental insufficiency

• Reduced nutrient transfer

• Altered fetal blood flow patterns

In Bangladesh, Rashid et al. (2017) found that high temperatures during early and late pregnancy were associated with reduced birth weight and shorter birth length. The association was strongest among women with low BMI, indicating that heat effects interact with maternal nutritional status.

Neurodevelopmental and Metabolic Effects

Emerging research suggests that in utero exposure to heat may affect the development of the brain and metabolic systems.

• Studies show links between prenatal heat exposure and later life outcomes, including:

  • Lower cognitive scores

  • Higher rates of school dropout

  • Higher adult BMI and waist circumference (Münz et al., 2023)

Mechanisms may include:

• Epigenetic modifications

• Hypoxia during fetal growth

• Altered stress hormone programming

Continuous monitoring in mice has shown that heat-exposed embryos display distinct temperature signatures and altered postnatal behavior (Smarr et al., 2016).

Timing and Population Vulnerability

Risk is influenced by:

• Timing of exposure: Critical windows include early neural development (weeks 3–8), placental formation, and third-trimester growth acceleration.

• Severity and duration of heat exposure

• Maternal health: Women with underlying hypertension, low BMI, or poor access to cooling are at greater risk

• Geographic region and occupation: Agricultural workers in tropical regions face cumulative risk across multiple pregnancies

Recap

• Prenatal heat exposure increases the risk of miscarriage, stillbirth, preterm birth, and low birth weight

• Long-term developmental and metabolic effects are emerging concerns

• The severity of risk is shaped by timing, maternal health, and access to mitigation strategies

References

• Basu, R., Malig, B., & Ostro, B. (2010). High ambient temperature and the risk of preterm delivery. American Journal of Epidemiology, 172(10), 1108–1117.

• Chersich, M. F., et al. (2020). Associations between high temperatures in pregnancy and risk of preterm birth, low birth weight, and stillbirth: systematic review and meta-analysis. BMJ, 371, m3811.

• Edwards, M. J. (1995). Hyperthermia and birth defects. Reproductive Toxicology, 9(5), 411–425.

• Münz, T., et al. (2023). Conception during cooler months improves metabolic health outcomes. Nature Climate and Health, 2(1), 14–27.

• Rashid, H., et al. (2017). Temperature during pregnancy influences the fetal growth and birth size. Tropical Medicine and Health, 45(1), 1–9.

• Shiota, K., & Opitz, J. M. (1982). Neural tube defects and maternal hyperthermia. American Journal of Medical Genetics, 12(3), 281–288.

• Smarr, B. L., Zucker, I., & Kriegsfeld, L. J. (2016). Detection of successful and unsuccessful pregnancies in mice within hours of pairing through core body temperature. PLOS ONE, 11(10), e0165127.