Scientists from the University of Granada used hair samples to monitor cortisol in a cohort of over 100 pregnant women and made a fascinating discovery: Women experiencing stress before and during conception are twice as likely to give birth to a girl than a boy.
Cortisol is a steroid hormone that is used as an indicator of stress levels. It is commonly measured in the blood, saliva, or urine, but analyzing cortisol deposits in hair can serve as a tool to track cortisol levels historically over time.
The team of researchers obtained hair samples from participants from before conception to week 9 of their pregnancies to identify if the stress was a determinant of the baby’s sex. They found that mothers of girls had nearly double the amount of hair cortisol concentrations than mothers of boys.
“The results we found were surprising, as they showed that the women who had given birth to girls presented higher concentrations of hair cortisol in the weeks before, during, and after the point of conception than those who had boys,” said study lead María Isabel Peralta Ramírez.
The study represents one of the first in the field to examine the complex physiological connections between stress and sex determination.
Though follow-up studies will need to be conducted to dig deeper into the mechanisms behind this curious observation, the authors propose two possible theories. One is that cortisol may modify the concentrations of sex hormones such as testosterone which could influence sex determination.
Secondly, cortisol levels in the mother may favor X chromosome-carrying sperm to penetrate the cervical mucus over those with Y chromosomes. The X chromosome sperm (that determines a baby will be female) are thought to be more resilient than their Y chromosome counterparts.
“There are other possible hypotheses that attempt to explain this phenomenon. Among the strongest theories is the idea that there are more terminations of male fetuses on medical grounds during the first weeks of gestation in situations of severe maternal stress. That said, in light of the design of these studies, it is recommended that the results are corroborated in greater depth,” commented Peralta.