Measured from the moment of conception, the ratio of male to female embryos is almost exactly 1:1. Yet, the ratio of baby boys to baby girls born each year fluctuates perceptibly over time. What forces are at play that swing the balance one way or the other? A new study, led by Andrey Rzhetsky of the University of Chicago, US, offers new insight into the factors that may – or may not – affect sex ratios at birth.
The intangible influences that sway this ratio to favour one sex over the other have long been the subject of speculation, with popular theories ranging from the changing of the seasons to the stress levels of expectant mothers. This new study illuminates the veracity of the influence of a number of these factors, and has narrowed the field to a select handful of promising candidates – including environmental pollutants.
The study, published in the journal PLOS Computational Biology, analysed more than 6 million birth records across the US and Sweden. The team utilised two staggeringly enormous datasets – one from IBM Watson Health MarketScan insurance claims, representing over half (150 million) of the US population over eight years, and the other from the Swedish National Patient Register, covering the entire Swedish population (9 million) for over 30 years.
By pairing sex data with information on weather, pollutants, and a host of social factors likely to induce stress, such as violent crime and unemployment rates, the team were able to test more than 100 hypotheses regarding the factors associated with skewed sex ratios.
The clear picture that emerged is that sex ratios are not associated with seasonality or fluctuating temperatures, but that a range of both air and water pollutants could be influential. These pollutants included polychlorinated biphenyls (PCBs), iron, lead, mercury, carbon monoxide and aluminium in the air, and chromium and arsenic in water.
What is perhaps less clear is the direction of the skew – some pollutants were associated with more baby boys, others with more baby girls. At a biological level, the sex ratio at birth can be affected by maternal hormones that can act to selectively terminate female or male embryos during pregnancy. But until direct mechanistic pathways have been established between individual pollutants and selective termination, the nature of their influence on sex ratios remains correlative rather than causative.
“Ideally, each SRB-pollutant association could now be followed up with experimental work using human cell lines to dissect the underlying mechanism,” Rzhetsky says.
Other factors found to have an association with sex-skew included extreme droughts, traffic fatality rates, industrial permits, and vacant units in an area. Interestingly, some significantly stressful events appeared influential whilst others did not – the 2007 Virginia Tech school shooting was associated with an effect, while Hurricane Katrina, which devastated New Orleans in 2005, was not.
In all cases the observed sex ratio skew was modest, and the researchers note that while the identified associations are statistically significant, they still need to be corroborated by empirical evidence. But they believe their results could encourage policymakers to “decide to make steps toward reducing environmental pollution.”