The Argentine pontiff, addressing some 800 leaders of female religious orders in Rome for the meeting of the International Union of Superiors General, spoke of the nuns' vow of chastity.
But he said it must be a "fertile" chastity, generating "spiritual children in the church."
"The consecrated woman is a mother. She must be a mother and not an old maid!" he said to the women, whose orders were from more than 70 countries.
Francis, 76, spoke in Italian, using the word "zitella," which means "old maid" or "spinster."
"Forgive me if I talk like this, but this maternity of consecrated life, this fruitfulness, is important!" Francis said.
"Be mothers, like the images of the Mother Mary and the Mother Church. ... You are icons of Mary and of the church," he said in remarks reported by British newspaper The Daily Telegraph.
The Jesuit pope also lashed out against "careerists" and "social climbers" who "use the church ... as a springboard for their interests and personal ambitions," saying they do "great damage" to the church.