Mankind has long known that temperature affects plants, animals, and many natural processes. Crops grow differently in warm weather, and some animals change their behavior when temperatures rise. Researchers have also examined whether heat during pregnancy can influence birth patterns in humans.
A large study examined more than five million births across 33 countries in sub Saharan Africa and regions of India. Researchers compared birth records with climate data to see whether higher temperatures during pregnancy were linked to changes in the sex ratio at birth. The sex ratio at birth refers to the number of boys born compared with girls.
Across the world, this ratio usually remains stable. On average, about 105 boys are born for every 100 girls. Because this pattern rarely changes, small shifts receive scientific attention. The study found that when temperatures during pregnancy were higher than about 20 degrees Celsius, the number of male births was slightly lower. The pattern appeared in both Africa and India, though the timing and possible explanations differed between regions. The obvious fact to note is that the heat can not change the fetal sex, its was studies as a factor for the survival of fetus during pregnancy.
How Heat May Affect Male Births
In sub Saharan Africa, the link between heat and fewer male births appeared early in pregnancy. The first trimester showed the strongest association. Researchers often refer to the fragile male hypothesis to explain this pattern. Some studies suggest that male fetuses are more sensitive to stressful conditions during early development. Under conditions such as illness, poor nutrition, or environmental stress, male fetuses may have a higher chance of pregnancy loss before birth.
Heat places stress on the human body. High temperatures can affect circulation and the body’s ability to regulate temperature. These changes may increase strain during pregnancy, especially in the early stages. In the study, higher temperatures during the first trimester were linked with fewer male births in several African countries. The pattern appeared more clearly among women living in rural areas, women with limited formal education, and mothers who already had several children. Living conditions may influence exposure to heat. In many rural areas, people work outdoors and may live in housing without cooling systems. This can increase heat exposure during pregnancy.
Why the Pattern in India Looked Different
The results from India showed a different pattern. The association between heat and fewer male births appeared later in pregnancy, mainly during the second trimester. This stage overlaps with the period when prenatal testing can reveal the sex of a fetus. In some parts of India, cultural preferences for sons have led to the use of sex determination tests and selective abortion.
The researchers did not conclude that heat changes fetal sex. Instead, they suggested that extreme heat may affect behavior or access to medical services. Heat waves can disrupt daily activities. They may reduce working hours, limit travel, and create financial strain. These conditions can affect access to private clinics where prenatal testing and procedures may occur.
The reduction in male births during hot periods appeared strongest among groups previously associated with higher rates of sex selective abortion. These included older mothers, women having their fourth or later child, and women without sons in some northern Indian states. Because the effect appeared during weeks 13 to 20 of pregnancy, the researchers considered social and behavioral factors as possible explanations for the pattern in India.
What the Study Means for the Future
The research combined large demographic survey datasets with detailed temperature records. By linking birth data with weather conditions during specific weeks of pregnancy, researchers examined how heat exposure might relate to birth outcomes. The study included millions of births. Large datasets allow researchers to detect small shifts in sex ratios that smaller studies may not identify.
The authors noted several limits. Survey data often lack detailed medical information about maternal health, pregnancy complications, or exact timing of pregnancy loss. These gaps limit direct investigation of biological mechanisms. Demographic data also cannot fully confirm behavioral explanations such as changes in abortion access or family planning decisions.
Other studies have linked extreme heat with preterm birth, lower birth weight, and changes in fertility patterns. Research on heat exposure during pregnancy is continuing as global temperatures rise. Pregnancy changes how the body regulates heat. Increased blood volume, metabolic activity, and hormonal changes affect temperature control. Because of this, researchers study how heat exposure affects maternal and newborn health.
The study does not show that temperature determines whether a baby is male or female. Sex is genetically determined at conception through chromosomes. The findings indicate that environmental conditions during pregnancy may influence which pregnancies continue to birth. Small shifts in birth ratios can become noticeable when they occur across large populations.
FAQs on High Temperatures Linked to Fewer Male Births
Q: Can high temperatures during pregnancy affect the chances of having a boy or a girl?
A: Some research suggests that exposure to high temperatures during pregnancy is linked with slightly fewer male births. The research does not show that heat changes the sex of a baby, but it may influence which pregnancies continue to birth.
Q: What does the sex ratio at birth mean?
A: The sex ratio at birth refers to the number of boys born compared with girls in a population. Globally, the average ratio is about 105 boys for every 100 girls. Because this ratio usually stays stable, even small changes can attract scientific attention.
Q: Why might male fetuses be more affected by environmental stress?
A: Some studies suggest that male fetuses may be more sensitive to difficult conditions during pregnancy. This idea is often called the fragile male hypothesis. Under stress such as illness, poor nutrition, or environmental heat, male fetuses may have a slightly higher risk of pregnancy loss.
Q: During which stage of pregnancy does heat exposure matter most?
A: The timing may vary by region and situation. In some African countries, higher temperatures during the first trimester were linked with fewer male births. In India, the association appeared later, mainly during the second trimester of pregnancy.
Q: Does heat directly determine whether a baby is male or female?
A: No evidence shows that temperature determines fetal sex. A baby’s sex is decided at conception through chromosomes from the sperm and egg. Heat may influence pregnancy conditions and outcomes, but it does not change the genetic sex of a fetus.
Q: Why were the results different in Africa and India?
A: Researchers found that the timing of heat exposure and social factors may explain the difference. In sub Saharan Africa, the pattern appeared early in pregnancy and may be linked to biological stress. In India, the association appeared later in pregnancy and may relate to access to prenatal testing and reproductive decisions.
Q: Can extreme heat affect pregnancy in other ways?
A: Yes, several studies have linked extreme heat to pregnancy health risks. These include higher chances of preterm birth, lower birth weight, and stress on the mother’s body. Researchers continue to study how heat exposure affects maternal and newborn health.
Q: Could climate change influence birth patterns in the future?
A: Rising global temperatures mean that more pregnancies may be exposed to periods of extreme heat. If the patterns seen in research continue, climate conditions could influence reproductive outcomes in subtle ways. Scientists are studying this issue as part of broader research on climate change and health.
Q: How can pregnant women reduce risks from extreme heat?
A: Health experts recommend staying hydrated, avoiding long periods in direct sunlight, and resting in cool environments during heat waves. Medical guidance and regular prenatal care can also help monitor health during pregnancy. Public health programs often provide heat warnings to help reduce risks for vulnerable groups.
External Sources:
- Abdel Ghany J, Wilde J, Dimitrova A, Kashyap R, Muttarak R. Temperature and sex ratios at birth. Proceedings of the National Academy of Sciences. 2026 Feb 24;123(8):e2422625123. Doi: 10.1073/pnas.2422625123.
- McCarthy MM. Stress during pregnancy: Fetal males pay the price. Proceedings of the National Academy of Sciences. 2019 Nov 26;116(48):23877-9. Doi: 10.1073/pnas.1916909116.
- News from Nuffield College, University of Oxford. New Research Shows High Temperatures Affect Sex Ratios at Birth. Available from: https://www.nuffield.ox.ac.uk/news-events/news/new-research-shows-high-temperatures-affect-sex-ratios-at-birth/
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