It has been studied that after situations of adversity, such as the 9/11 attacks, there is a greater birth of girls
There are two types of hypothesis: some focused on the behavior of sperm and others on the role of stress in spontaneous abortions.
Psychological stress is, without a doubt, one of the greatest villains of the last century. However, despite the dire consequences it can have for health, its ultimate function is to save our lives as a species.
This is supported by the recent results published in the Journal of Developmental Origins of Health and Disease. There we found that women who give birth to a baby girl have higher levels of cortisol (stress hormone) at the time of conception. So much so that they have twice the amount of cortisol in the case of being a girl.
Stress affects sex hormones
Nature has its own mechanisms in adversity situations. First of all, it is important to note that sex hormones can be modified in high stress situations. An example of this is the reduction of testosterone in men with high levels of stress, as seen in soldiers at war or in college students after performing a stressful task.
Moreover, in athletes who are subjected to overexertion (physical stress) not only a decrease in testosterone has been found, but also a lower mobility of sperm.
As for women, the perception of psychological stress is related to a decrease in estrogen and progesterone, the hormone responsible for maturing the walls of the uterus so that the ovum implants. Simultaneously, with stress prolactin is triggered, which in turn is related to the inhibition of ovulation.
Studies carried out in Nazi concentration camps showed that 54% of women stopped menstruating in the first month (at which point they still had the necessary body fat). This hormonal modulation due to stress is also related to the woman being less receptive and sexually proactive.
Why after the 9/11 terrorist attacks, more girls were born
According to the World Health Organization (WHO), in the human species the relationship between men and women at birth is in favor of the male sex with a ratio of approximately 105 or 106 male births for every 100 female. However, this can be modified under certain conditions.
It has been studied that after situations of adversity –exposure of the population to extraordinary stressful events– there is a greater birth of girls. This has happened at various times in history, for example in the United States in the months after the assassination of President JF Kennedy, after the 9/11 terrorist attacks or after the death of Princess Diana in England.
There are two types of hypotheses that try to explain this phenomenon. Some are centered on the sperm behavior carriers of the X and Y chromosomes (which define the sex of the baby). They defend that the sperm carrying the X chromosome (which when joining an egg will form a girl fetus) are more resistant, crossing cervical mucus with a more acidic pH (and more hostile).
It makes sense considering that the pH is more acidic when estrogens decrease as a result of stress. In this way, in the face of adversity, the sperm carrying the Y chromosome (which when joining an egg will form a child fetus) would have less chance of reaching said egg.
On the other hand, there is ample scientific evidence on the role of stress in spontaneous abortions. In fact, different studies have shown an increase in the number of abortions after dramatic episodes such as the 9/11 terrorist attacks or after the East Japan earthquake in 2011. Well, it turns out that different authors have come to the conclusion that that these miscarriages occur selectively in higher numbers in male fetuses (XY).
More options for the X
Most likely, the increased birth of girls in stressful situations is due to a conjunction of these two phenomena. It is not only that the sperm carrying the X chromosome are more resistant crossing the cervical mucus. Furthermore, in adverse circumstances, once the ovum is fertilized, the XX fetus is more likely to survive a miscarriage.
Is this response to stress in the face of adversity an adaptive response with which nature ensures the survival of the species? And after this devastating COVID-19 pandemic, will more girls be born?
This article was originally published on The Conversation. Read original here.