A policy of only having sex when the female of the relationship isn't ovulating (which you can easily determine with a calendar, a sextant, and a pot of boiling water). "Pulling out" is also a popular method of family planning without the expense, hassle, or security of proper birth control.
See also: pregnancy.
"Yeah, we've been using the rhythm method for a while now."
"And how old is little Bucky?"
"He'll be two by the time his sister is born in August."
LMFAO clay with the one hander! HAHA got to love it.