menstruation & pregnancy

Hi --

It seems like my questions are getting weirder and weirder ... or, is that: "curiouser and curiouser"?

In any case -- I need to know when, approximately, it became common knowledge that menstruation and pregnancy were intrinsically linked. In particular, when would the "rhythm" method (i.e. calculating fertile days from the date of a woman's last period) be in widespread use in China? The United States?

Would it be common knowledge by 1900 or so?

Thanks --

Perfect!

Thanks, Birol --

It says:

The rhythm methods (based on calculating the woman’s fertile period and abstaining from intercourse during it) were widely discussed during the 19th century. Unfortunately it was very ineffective during the 19th and early 20th century, since the female fertility cycle was not understood until 1920. Until that time, observing other mammals lead most to believe ovulation occurred either during menstruation or just before it.

Perfect! My character does, indeed, get pregnant.