Speaking during a press conference on his flight home from a trip to the Philippines he said it was important to practice 'responsible' parenting.
He had visited street children, whose parents had abandoned them because they couldn't afford to pay for them, whilst visiting the country.
His statement is not a change in Catholic doctrine on birth control; he added that new life was "part of the sacrament of marriage".
He said: "Some people think that - excuse my expression here - that in order to be good Catholics we have to be like rabbits.
"No. Parenthood is about being responsible. This is clear."
The Holy Father said there were many ways good Catholics could plan their families without using birth control.
He spoke of meeting one woman who had seven children by caesarean section and was expecting her eighth.
He said that pregnancy was irresponsible: "She said, 'I trust in God.' But God gave us the means to be responsible."