Finland’s babies did not appear to change their biological balance between boys and girls when the country confronted two major crises in quick succession: the COVID-19 pandemic and Russia’s full-scale attack on Ukraine. A nationwide study covering 25 years of monthly birth records found no convincing evidence that either emergency altered the proportion of male live births or male stillbirths. The result offers a rare population-scale look at how reproductive outcomes responded to overlapping health, economic and geopolitical shocks—and suggests that Finland’s birth sex ratio remained remarkably stable despite extraordinary social disruption.
Researchers Samuli Helle, Antti O. Tanskanen and Mirkka Danielsbacka analyzed monthly data from Finland’s Medical Birth Register between January 2000 and December 2024. The dataset included 1,355,037 live births and 4,096 stillbirths, making it one of the largest studies to examine both living and stillborn infants in relation to major external crises. In the Finnish register, a stillbirth is defined as a fetus or newborn showing no signs of life after a pregnancy lasting at least 22 weeks or involving a birth weight of at least 500 grams. The investigators examined whether the fraction of male births shifted immediately after the crises or during the following 13 months.
At the population level, the human sex ratio at birth is usually slightly male-biased. In Finland, males represented approximately 51.2 percent of live births during the study period, while the proportion of male stillbirths averaged about 51.0 percent. These percentages fluctuate naturally from month to month, particularly when the total number of births is small. Live births in the Finnish data ranged from 3,276 to 5,472 per month, but monthly stillbirths ranged from only four to 28. That difference makes stillbirth ratios especially noisy: a change of just a few cases can produce a dramatic-looking percentage shift even when no underlying population-wide change has occurred.
The scientific question arises from earlier research suggesting that severe stressors may influence the sex ratio through several biological and behavioral pathways. Male fetuses are sometimes considered more vulnerable to adverse conditions during development, potentially leading to greater male embryonic or fetal loss during periods of maternal illness, nutritional stress or psychological strain. Chronic stress can also affect glucocorticoid hormones, including cortisol, which may influence pregnancy physiology. A separate hypothesis proposes that social disruption changes conception patterns or sexual behavior, potentially producing a different sex ratio roughly nine months later. These ideas are often discussed in connection with the Trivers–Willard hypothesis, which predicts that mothers in better condition may benefit more from investing in sons, while mothers facing adversity may favor daughters. Whether this evolutionary framework reliably applies to humans remains controversial.
COVID-19 created a natural test of these theories, although not a perfectly controlled one. The pandemic affected health, employment, mobility, family life and access to medical services, while the intensity of exposure varied considerably between individuals and regions. Previous studies from England and Wales, South Africa, Japan and Ireland have reported declines in the proportion of male births at different delays after the pandemic began. Other investigations in Croatia and the United States found no statistically significant change, and a study from Mexico reported an increase during part of the pandemic. The inconsistent international record made Finland an important case, especially because the country has high-quality registers, relatively strong public institutions and a comprehensive welfare system that may have buffered some of the pandemic’s effects.
The researchers used Bayesian aggregated binomial time-series regression rather than treating monthly percentages as if they were equally precise measurements. In this approach, the number of male births is modeled as the number of events, while the total number of births supplies the number of opportunities for those events to occur. A month containing 5,000 births therefore contributes more statistical information than a month containing five stillbirths. The models incorporated the immediate effects of the COVID-19 outbreak, dated to March 2020, and Russia’s full-scale offensive against Ukraine, dated to March 2022 because the invasion began near the end of February. The analysis also tested effects delayed by one to 13 months, while accounting for seasonal patterns and long-term changes in the birth sex ratio.
The results were clear for the pandemic: none of the tested COVID-19 time lags showed credible evidence of a change in the proportion of male live births. The male share also appeared relatively stable after the pandemic began, without the pronounced monthly spikes seen at some earlier points in the 25-year record. The same conclusion applied to male stillbirths. When the researchers treated the entire 13-month COVID-19 period as a single exposure rather than examining individual delays, the estimated odds ratio for male live births was 0.996, with a 95 percent credibility interval from 0.978 to 1.015. In practical terms, the estimate was extremely close to no change, and the interval included values compatible with both a small decrease and a small increase.
The analysis did identify one unusual signal after Russia’s attack on Ukraine. Four months after the beginning of the offensive, in July 2022, the odds of a male live birth were estimated to be 7.9 percent higher than expected, with a 95 percent credibility interval of 1.4 to 14.8 percent. July 2022 was one of the months with the highest proportion of male live births in Finland during the century examined. However, the authors caution strongly against treating this isolated result as proof that the war caused a biological shift. The researchers tested numerous lags, and a single statistically notable association can arise by chance when many possible time windows are examined. Moreover, a rise in male births four months after a crisis is difficult to reconcile with the most commonly proposed mechanisms, which generally predict increased male fetal loss or effects near conception.
An alternative timing clue comes from Finland’s recovery from pandemic restrictions. The country lifted all restrictions on gatherings on October 1, 2021, approximately nine to ten months before the July 2022 birth peak. The authors speculate that a short period of renewed social contact and optimism might have influenced conception patterns, although this explanation is also unproven. The timing could reflect a post-restriction change in sexual behavior, seasonal variation, an unrelated demographic fluctuation or simple statistical coincidence. The study found weak seasonality in live births, with the male proportion generally highest during summer months, and a long-term pattern in which the proportion declined from 2000 and began rising again after about 2016. These background movements make it especially important not to interpret one unusual month as a direct consequence of war.
No convincing association with either crisis appeared in the stillbirth analysis. The researchers found no non-zero COVID-19 or war-related coefficients across the tested lags, although the small number of stillbirths limited statistical precision. The study also found that male live births and male stillbirths were not meaningfully correlated after adjustment for seasonal and long-term patterns. Sensitivity analyses using different prior assumptions produced broadly similar conclusions, and beta-binomial models designed to account for extra variation did not improve predictive performance. Statistical diagnostics indicated that the Bayesian models converged successfully and reproduced the observed data reasonably well.
The findings do not mean that COVID-19 or the war had no reproductive consequences. A stable national birth sex ratio can conceal substantial differences between groups. The pandemic and the economic consequences of the war may have affected people according to occupation, income, education, region, health status and family circumstances. The study used nationwide monthly counts and could not measure individual stress, infection during pregnancy, vaccination status, economic hardship or access to care. It also could not distinguish whether a change in sex ratio would have arisen through conception behavior, pregnancy loss, stillbirth or other mechanisms. A population-wide crisis indicator is only an indirect and imperfect measure of the stress actually experienced by pregnant women and couples.
The authors therefore describe Finnish society as resilient in terms of these broad reproductive indicators, rather than claiming that every family experienced the crises without harm. Finland’s institutional trust, public healthcare system and economic protections may have reduced the extent to which collective emergencies translated into changes in birth outcomes. Earlier research has found that fertility behavior in Nordic countries during the pandemic was sometimes shaped more by economic security and confidence in institutions than by the crisis alone. The researchers argue that future work should use individual-level records and examine longer-term outcomes, including child health, development and sex-specific effects among children born during periods of upheaval. For now, the central message is restrained but significant: across more than 1.35 million Finnish live births, neither the pandemic nor Russia’s attack on Ukraine produced a robust, population-wide shift in the proportion of boys and girls born.
