Is NFP based on guesswork?

Not at all. NFP is based on scientific facts about fertility. The methods are developed from research about women's menstrual cycles and the signs of female fertility.

Over a century ago, scientists discovered cyclic changes in cervical mucus and their relation to ovulation. In the 1920's, scientists identified the temperature rise that signals ovulation. But it was not until the 1950's that scientists developed programs to teach others how to observe and interpret these fertility signs. Today, ongoing research continues to refine the methods of Natural Family Planning.