My particular church requires all engaged couples to take Natural Family Planning. The first lesson is basic reproductive biology, and I'm surprised at what some people don't know.
For example, ovulation only lasts one day. If the egg isn't fertilized in 24 hours, it instantly dies. I thought everyone knew that! But it turns out that most people think the egg is alive for the entire two-week period between a woman's ovulation and her period. Not true! The window for getting pregnant is actually pretty small. (The window for impregnating someone is larger, since the man's seed lasts 3-5 days under optimal conditions.)
There are a few signs that indicate a woman is ovulating. First, there is a rise in temperature. Second, the cervical lining becomes more porous. There are also hormonal changes, but those two are the ones that you can easily keep track of, without the help of a doctor.
Lesson #2 is next month, and I wonder what we'll learn.