Besides the immorality of contraception (CCC 2399), what does contraception have to do with being pro life?
Many forms of birth control, are abortifacients, meaning that the device or substance is capable of unnaturally ending the life of a preborn child. Since 1973, over 61 million preborn children have lost their lives from medical and surgical abortions but these numbers do not include those killed because of the use of contraceptives. It is estimated that an additional 610 million preborn children died from the use of contraceptives between 1965 and 2009.
According to the Guttmacher Institute, 51% of U.S abortion patients report using contraception the month they got pregnant.
For more information on contraception and its ties to abortion.