Stress: Nature's Gender Bias?

- The Cortisol Connection: The surprising science linking maternal stress to a baby's gender.

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Stress: A household name and a silent killer, does it also play a role in determining your baby's gender? Science says yes! For ages, we have believed that the sex of a baby- either male or female- is totally up to chance, and this is true for the most part, but recent developments in our understanding of the role of extraneous variables have led to great breakthroughs in the science of sex determination.

In this blog, we will be focusing on research done that links high stress levels in mothers with the birth of more female than male babies. Can female fetuses handle stressful environments better than male fetuses? Let’s try to understand the various aspects of this.

Sex determination in humans

What is used as an index for stress levels, and how is it measured?

A hormone known as cortisol is used as an index for stress levels. Cortisol levels rise in individuals who are highly stressed. Cortisol levels were analysed from the hair cells of women in weeks 8-10 of pregnancy. This is indicative of cortisol levels over the last three months (one month per centimeter of hair growth), meaning that it covers the period before and after conception of the fetus.

Cortisol and its functions

What were the results obtained?

The researchers found rather surprising results, as they showed that the women who had given birth to girls presented higher concentrations of hair cortisol in weeks before, during, and after the point of conception than those mothers who had given birth to boys. In fact, the cortisol concentration in the hair cells of mothers who had given birth to girls was almost double that of those who had boys!

Results Obtained

Why were these results obtained?

• A possible explanation for the results would be the activation of the body’s “stress system”, also known as the hypothalamo-pituitary-adrenal axis. What sounds like a long and confusing name is an indispensable system that regulates the release of stress hormones in the body, such as cortisol. It is believed that an increase in cortisol secretion modifies the concentration of sex hormones (testosterone and oestrogen) during conception. 

  
  

  

  Hypothalamo-pituitary-adrenal axis

• There is also scientific evidence that sperm carrying the X chromosome (which determines if the baby will be female) are better equipped at passing through the mucous plug created in the female reproductive tract due to hormonal changes associated with stress in the mother than those carrying the Y chromosome.

Conclusion

Maternal stress has a key role to play in determining the sex of the fetus. Although there is still a considerable amount of debate over the different ways in which stress affects the sex ratio, a clear link between the two has been established. With advancing equipment and research techniques, the future possibilities are endless.

References

Romero-Gonzalez, B., et al. (2021) Hair cortisol levels in pregnancy as a possible determinant of fetal sex: a longitudinal study. Journal of Developmental Origins of Health and Disease. doi.org/10.1017/S2040174420001300.