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Stress leads to premature births and increases girls' fertility rates
Last reviewed: 01.07.2025
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According to a study published in the journal Human Reproduction, mothers who are stressed during the second and third trimesters of pregnancy have an increased risk of preterm birth. In addition, stress can affect the sex of the baby, leading to a decrease in the birth rate of male babies.
The study looked at the impact of stress caused by the 2005 Chile earthquake on pregnant women.
Scientists have known for some time that stress can shorten the duration of pregnancy, but this is the first study to examine the impact of stress on the ratio of boys to girls born.
In Chile, between 2004 and 2006, more than 200,000 births were registered per year. Birth certificates of all babies born during this period were examined by scientists Florencia Torche and Karine Kleinhaus from New York University (USA).
Each birth registration certificate contained data on gestational age at birth, weight, height, and sex of the child. In addition, data on mothers' age at birth, previous pregnancies, and marital status were collected.
"By looking at gestational age information in a large group of women at the time of the earthquake, we were able to determine how stress affects women at different gestational ages depending on how close they were to the earthquake's epicenter," said study author Florenci Torche.
A team of scientists found that women who lived closest to the earthquake's epicentre during the second and third trimesters of pregnancy had a higher risk of preterm birth.
About 6 out of 100 women had a preterm birth. Women exposed to the earthquake in the third trimester of pregnancy had a 3.4% higher risk of preterm birth.
The effect of stress on pregnancy was most pronounced for girls, with the likelihood of preterm birth increasing by 3.8% if the mother was in the third trimester and 3.9% if she was in the second trimester. No statistically significant effect was observed for preterm births of boys.
When analyzing the impact of stress on the ratio of boys to girls born, the scientists found that earthquake stress had a greater impact on the preterm birth of girls.
Typically, the ratio of boys to girls is about 51:49. In other words, out of every 100 births, 51 are boys. Scientists' data showed a 5.8% decline in this proportion, which translates into 45 boys born per 100 children born.
The scientists' findings about the decreasing sex ratio at birth support the hypothesis that stress may affect the viability of boys during the prenatal period of development.
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