Casti connubii (Latin: "of chaste wedlock")[1] is a papal encyclical promulgated by Pope Pius XI on 31 December 1930 in response to the Lambeth Conference of the Anglican Communion.

It covered four major topics: the sanctity of marriage, opposition to eugenics, positions on birth control and the purpose of sexuality, and reaffirmation of the prohibition on abortion.

This ... does not deny or take away the liberty which fully belongs to the woman both in view of her dignity as a human person, and in view of her most noble office as wife and mother and companion; nor does it bid her obey her husband's every request if not in harmony with right reason or with the dignity due to wife; ... For if the man is the head, the woman is the heart, and as he occupies the chief place in ruling, so she may and ought to claim for herself the chief place in love.

[4]Casti connubii speaks out against the eugenics laws, then popular, that forbade those deemed 'unfit' from marrying and having children: "Those who act in this way are at fault in losing sight of the fact that the family is more sacred than the State and that men are begotten not for the earth and for time, but for Heaven and eternity.

[10] However, a few Catholic theologians continued to hold that such practices were equivalent to contraception and thus immoral, and some historians consider two 1951 speeches by Pope Pius XII[11] to be the first explicit church acceptance of natural family planning.

In a 1932 article published in The Nation, Margaret Sanger gave her personal reaction to the encyclical, saying that it was an obstacle to general approval of the birth-control movement by political leaders unwilling to oppose the leadership of the church.