https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1119278/

The human male is, on most measures, more vulnerable than the female. Part of the explanation is the biological fragility of the male fetus, which is little understood and not widely known. A typical attitude to boys is that they are, or must be made, more resilient than girls. This adds “social insult to biological injury.”

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1119807/

https://www.livescience.com/574-survival-fetus-males-rough.html

During times of social or economic stress, a woman's liver tends produces more of a hormone called cortisol that proves so damaging to male fetuses they actually kick out in response to it. Female fetuses, more vital on the whole, seem relatively unaffected by the cortisol. One theory states that damage to male fetuses is a side effect of this hormonal stress response. But in a new study, researchers provide evidence for the other theory, that the body is purposely culling the males by pumping out cortisol in an effort to get rid of a child-to-be that is less likely to survive the presumably difficult situation outside the womb.