Catechism of the Catholic Church §2379 — excerpt
The Gospel shows that physical sterility is not an absolute evil. Spouses who still suffer from infertility after exhausting legitimate medical procedures should unite themselves with the Lord's Cross, the source of all spiritual fecundity.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.