My son once remarked that the incidence of secret female soldiers pretty much has to be higher than we suspect because, by definition, if they succeeded, everyone thought they were male. If you died on the field of battle during a bad battle, you might be left there or buried in a mass grave. You didn't necessarily get an individual burial.

Re amenorrhea. Being underweight and exercising a lot can cause amenorrhea.*

In Canada, they did an experiment to let women into the infantry on the condition that they meet the same standards as men. Only two women made it through. One quit before being assigned to a unit. The other quit a few weeks after getting assigned. She said she thought it would get easier once she was in a unit. She thought training was intentionally harder than normal routine for the military. But it never got easier.

Unlike male firefighters, female firefighters have to lift weights to be able to do their job. A big guy can do a fireman's carry without being a body builder. This is generally not true for women.

I have serious health issues. As a teen, my periods tended to be 3 to 8 months apart. I was quite skinny when I first began menstruating. My belief is that my irregular periods were largely rooted in how underweight I was. I also was nearly 17 when I began menstruating.

So I strongly suspect that women who successfully passed themselves off as male in very physically demanding jobs probably had their periods suppressed to at least some degree. The very act of trying to physically keep up with the demands of the job may have helped hide their gender by reducing or eliminating their periods.

In times and places where sex outside of marriage was very much frowned upon, it wouldn't necessarily be weird to just be celibate. This would further help hide the person's sex, both by not exposing their body to another and by eliminating the risk of pregnancy.

This also was an era that was not awash in information like we are today. If a baby was born with an oddity, it might be the only time anyone dealing with the child had seen or heard of such a thing. Even physicians had limited information compared to what you can google up in a few minutes these days.

In my youth, I was told at times that I was extremely feminine. One woman who played baseball told me she couldn't imagine me playing a sport because I was so very feminine. (I actually did gymnastics for a time, though not competitively.)

After being homeless for a bit, I began getting addressed as "sir" by people who saw me from the back. I'm tall for a woman and had short hair, but both things had been true before I was homeless and no one mistook me for a man.

I suspect two things made the difference:

1. I stopped wearing a bra. Bra straps are visible through clothing, even from the back, and signal gender.

2. I think my demeanor and body language changed.

So, if you dress in men's clothes and adopt male body language, I think that goes a long way towards convincing others you are male.

Also, I was extremely flat chested until I had my first baby. I gained a cup size with each of my two pregnancies, putting me at an average size. Talking to other women, this seems to be pretty normal.

So it seems if you don't ever have a baby, you may not become very busty. Being flat chested definitely makes it easier to pass for male.

* https://healthfully.com/486066-underweight-and-amenorrhea.ht...