Every pregnant woman in India has heard some version of a dadi’s prediction. Heartburn means a hairy baby. Carrying low means a boy. Carrying high means a girl. Most of this is harmless folklore. But scientists have found something that sounds like folklore yet appears to be true. After wars, famines, pandemics, earthquakes, and financial crashes, more baby girls are born. The pattern has appeared repeatedly in birth records. It spans different countries, decades, and disasters. So, does stress affect a baby’s gender? Yes. But scientists are still working to understand exactly why.

Nature already favours boys

Before getting into crises, it helps to know what “normal” looks like. Across the world, roughly 105 boys are born for every 100 girls. It’s a small, consistent male excess that’s held steady for as long as records exist. One theory is that this buffers against boys’ higher rates of infant mortality and risk-taking later in life, so the numbers even out by adulthood. So when researchers talk about “more girls being born,” they mean it happens for a window of months during and after a crisis.

Does stress affect gender of a baby?

The clearest illustration comes from the United States between 1918 and 1939, a period that captures a pandemic, a boom, and a depression. Researchers examined sex ratios across the Spanish Flu, the Roaring Twenties, and the Great Depression. They found that fewer sons were born during the Spanish Flu and the Great Depression. During the prosperous Roaring Twenties, more boys were born, with the ratio rising to 117 boys for every 100 girls. The researcher behind the study, quoted in an East Carolina University report, stated that during bad times, noticeably fewer boys are born than during good ones.

This isn’t a one-off finding. After the September 11, 2001 attacks in the US, a well-cited study of over 700,000 births in New York found the city’s birth sex ratio dropped to its lowest recorded level exactly four months later. That is the point at which fetuses conceived just before, or in early pregnancy during the attacks, would have been born. A review in the journal Early Human Development found the same dip following the violence in Northern Ireland, riots in Los Angeles, and mass shootings.

There’s also individual-level evidence, not just population-wide trends. A Columbia University study tracking real pregnant women found that those under physical stress like high blood pressure had roughly four boys for every nine girls, while psychologically stressed mothers had two boys for every three girls. 

Why does this happen?

There are two competing (and possibly complementary) explanations. Male fetuses are the more fragile sex, and stress makes it worse. Contrary to the tough, hardy image often attached to boys, male fetuses are biologically more delicate than female ones from conception onward. Research from the University of Adelaide, tracking over 574,000 births, found boys face significantly higher risks of pre-term birth and complications than girls at every gestational stage. Under normal conditions, enough male embryos survive to produce that familiar 105:100 ratio. Under stress, the more fragile male fetuses are lost before birth, slightly nudging the ratio towards girls. The 9/11 study specifically concluded this was driven by “male fetal loss” rather than couples somehow conceiving fewer boys to begin with.

Evolution may have wired parents to do this on purpose. This is the more provocative idea, known as the Trivers–Willard hypothesis, first proposed in 1973. The logic is that a strong son born into good conditions can have many children and spread his genes wide, but a weak, struggling son might father none at all. A daughter’s chances of having children, on the other hand, don’t swing nearly as much based on her circumstances. So the hypothesis predicts that under poor conditions, Mother Nature favours daughters because they have a genetically superior payoff.

However, it’s a widely contested idea, and not every crisis produces the effect. A 2024 US study on COVID-19 found no overall national dip in male births, even though the pandemic was about as universal a stressor as it gets; though it did find fewer boys born specifically to mothers from more disadvantaged backgrounds, suggesting the effect may depend on how acutely a crisis is actually felt, not just whether it happened.

What about India?

The most direct Indian evidence comes from the 2001 Gujarat earthquake, which killed more than 20,000 people. A study published in the Journal of Population Economics compared District-Level Household Survey data from earthquake-hit districts against neighbouring, unaffected ones, before and after the quake. It found the disaster measurably reduced male births among rural women in the affected areas. This Indian data point lines up with the global pattern.

But here’s the catch, and it’s a big one. This natural shift toward girls is outweighed by a preference for sons and sex-selective abortion in India. As a result, India’s sex ratio at birth has remained above the natural range for decades. Pew’s report estimated that roughly 4.1 lakh girls “go missing” at birth in India every year as a result, a figure that has fallen from 4.8 lakh in 2010 but remains staggering.

Biology alone won’t decide the answer

There isn’t yet a large, India-specific study on how COVID-19 affected the country’s sex ratio at birth the way the Gujarat earthquake did. However, Indian researchers have studied in detail how COVID-19 widened gender gaps in schooling and literacy. A 2023 study in the journal Nature’s affiliated PMC archive found Indian girls lost more schooling years than boys after the pandemic, with caste and poverty compounding the disadvantage. It reminds us that in the Indian context, the more urgent and better-documented gender story during crises is not who gets born. It is who stays in school, who gets fed first, and who gets taken to the doctor once she is here.

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