The terrorist attack of September 11th, 2001, deeply traumatized America. Even people watching on TV thousands of miles from the actual incident were terrified that such a thing could happen in their town too. And, to everyone's surprise, demographic studies showed that it caused thousands of miscarriages nationwide. Even more curiously: all the miscarriages would have been male had they been born. Wait, what?
Many women speak enviously of some male traits. Men are stronger, having 3 times the muscle density, easier time working out, easier time losing weight, and make more money. But this comes at a cost.
Maleness is, essentially, fragile. We humans are conceived with a male:female ratio at about 1.6:1, and this is down to about 1.2:1 by birth, and is down to 1:1 by the time we're 30. If you're male, you're more likely to abruptly die. Both unexpected incidents, like heart disease, and injuries from unwise events ("Hey guys, I'm going to jump this bike over this set of boxes! Oh crap, not far enough!") disproportionately affect men. Men's life expectancy is 4-7 years less than women's in all but the most misogynist of societies.
Even on a genetic level, maleness is fragile. Women have two X chromosomes, men have one X and one Y. X contains many things that you need to live, and deficiencies in yours can result in conditions from hemophilia to color blindness. X-linked conditions affect men way more than women, because women can use the secondary X chromosome to back up the first, while men only have one to work on. For a woman to be color-blind, she needs 2 X chromosomes with that trait, while a man only has 1 to work with anyway. Also, genetic errors can copy or delete chromosomes, and it's been noted that men who have more Y chromosomes (XYY, XYYY, etc) are more likely to be arrested, men who have X duplications (XXY, XXXY) only have a tendency towards being fat, transsexual, or both. Having only Ys (YY) results in an early miscarriage, as a fetus with that genetic set can't survive. While having even one Y chromosome makes you male, you need at least one X chromosome to live.
Biologists speculate that then in times of stress, this suggests danger to the tribe. The tribe will therefore need more females to repopulate than males (who would primarily just shuffle the genes and compete with each other), and maleness is temporarily selected against.
These biologists unfortunately don't have any advice for women wishing to avoid miscarriages. Not all stress can be avoided.