Long-term stress during pregnancy is well known to be unhealthy for both mother and child – but researchers have also long suspected that stressed women are more likely to have daughters than sons. Spanish scientists now even argue that pre-fertilization stress affects the child’s sex. The reasons for this are not yet clear, but there are several hypotheses.

It’s common knowledge that long-term stress is bad for our health. It is also known that stress in pregnant women can also be detrimental to the development of the baby. Too much tension can lead to the baby’s birth weight being too low, increases the risk of preterm birth and can cause babies to be sick more often in their first year of life.

Scientists have also long believed that long-term stress reduces the chances of birthing a boy. For example, the number of newborn boys after a violent event was found to be lower than normal. In the months after the 9/11 terrorist attack, for example, fewer boys were born in New York. Research into this phenomenon often focuses on the stress experienced during pregnancy, but a study by the University of Granada also recently looked at the influence of stress prior to fertilization.

Twice as likely

The Spanish researchers studied the amount of cortisol, the stress hormone, in the hair of 108 women who were about nine weeks pregnant. They were able to determine the stress levels of the women in the previous three months, i.e. from a few weeks before fertilization. They also subjected the participants to various psychological tests. That whole analysis found that long-term stressed women were about twice as likely to have a daughter.

Better handling of obstacles

But how could stress help determine a child’s gender? The Spanish scientists don’t have a final answer for this question, but they do give some hypotheses. For example, this phenomenon could be explained by the fact that sperm with an X chromosome (female babies) can better cope with adverse conditions during the journey to the egg – to fertilize them. Since stress can indirectly create certain obstacles on this pathway, those sperm in stressed women would have an advantage over counterparts with a Y chromosome (male babies).

More vulnerable male fetuses

On the other hand, it is also possible that male fetuses are more likely to be aborted in the first weeks of pregnancy for medical reasons, as they would be more sensitive to the effects of stress. Although there is a debate about this, it seems that male fetuses are more vulnerable to complications during pregnancy and birth. Thus, female fetuses would have better chances of surviving the effects of long-term stress in the womb. The Spanish team believes that this possible explanation deserves more research soon.

It is important to keep in mind that you do not have to panic when you are stressed. We all regularly deal with stress, but short-term stress does not have serious health consequences. It is only when you are continuously stressed for a long period of time that it carries serious risks to your well-being and that of your unborn baby.

Elena Mars

Elena writes part-time for the Scientific Origin, focusing mostly on health-related issues.