Toni Weschler

Has taught fertility awareness since the 1980s, and her book explains a cycle through observable signs instead of a calendar count.

Taking Charge of Your Fertility covers cervical fluid, basal body temperature and cervical position, and the point is that ovulation can be identified while it is happening rather than guessed at afterwards. It is used both to conceive and to avoid conceiving.

Weschler holds a master's in public health and is clear about what the method requires: daily charting, and a willingness to abstain or use barriers during a defined window. Done casually it is considerably less reliable than the figures suggest.