According to the Catechism of the Catholic Church:

(2363) The spouses’ union achieves the two-fold end of marriage: the good of the spouses themselves, and the transmission of life.  These two meanings or values of marriage cannot be separated without altering the couple’s spiritual life and compromising the goods of marriage and the future of the family.

(2367) Called to give life, spouses share in the creative power and fatherhood of God.  Married couples should regard it as their proper mission to transmit human life and to educate their children…They will fulfill this duty with a sense of human and Christian responsibility. 

(2368)  A particular aspect of this responsibility concerns the regulation of procreation.  For just reasons, spouses may space the births of their children…in conformity with the generosity appropriate to responsible parenthood…conforming their behavior to the objective criteria of morality.

(2370)  …The method of birth regulation based on self-observation and the use of infertile periods, is in conformity with the objective criteria of morality.  These methods respect the bodies of the spouses, encourage tenderness between them, and favor the education of an authentic freedom.

(2371) …Human life and the duty of transmitting it  are not limited by the horizons of this life only: their true evaluation and full significance can be understood only in reference to man’s eternal destiny.