My mother was a fan of the rhythm method, better known as the natural family method of birth control. Based on her experiences, as well of those of many of my friends, I have lumped everyone I know who has used that method under one term- Parents.
The rhythm method is a form of natural birth control and is approximately 75% effective. A woman using this method tracks her menstrual cycle on the calendar to determine her cycle length and then uses this information to avoid sexual intercourse on days when she is most likely fertile.
The rhythm method does not work. In theory, the man removes his penis before ejaculating and this prevents any pregnancy. In fact, this is very difficult to do correctly. Moreover, men often have a small amount of sperm that comes out before they orgasm, and this method does not protect against this.