Cel - PLEASE do not follow the 10 day rule unless you are prepared to be the tired mom of four!
This is based on the old "rhythm method" used during the 1950s when birth control was not as available as it is today. My two sisters and I (all born in the 50s) are all products of the "rhythm method" 10 day rule "birth control" method.
Women each ovulate at the time the body clock schedules each month (different for different women), and for those 2-3 days of peak fertility, it is not "SAFE" to have unproducted intercourse unless you are trying to have a baby, in which cause it is the BEST time to have unprotected sex. Women's body clock scheduling of ovulation differs - I happen to ovulate later (day 16-18ish) and many women ovulate earlier (day 10-12ish). Variables in life can sometimes throw this schedule off, so never have unprotected intercourse if you are of child-bearing age and and not trying to get pregnant.
For women trying to become pregnant, they can use a "basal body temperature" thermometer and chart to chart their monthly ovulation cycle to know optimal timing of intercourse to conceive! Thanks for the interesting topic, Cel! V. G. :o)