Personally, if my own wife's life was at stake, she and I would both choose Natural Family Planning to avoid pregnancy. Even secular medical journals have admitted its effectiveness. For example, the British Medical Journal reported, "Indeed, a study of 19,843 poor women in India [practicing NFP to delay pregnancy] had a pregnancy rate approaching zero."
It is a good intention to prevent pregnancy if the woman's life is at risk. That's not in question. The issue is the means that a couple might use to protect her health. NFP will do that more effectively than most contraceptive methods, and without the potential side-effects that all forms of contraception carry. It is the better, safer alternative. But even if contraception were more effective than NFP and posed no health risks, it is still intrinsically immoral, and therefore can never be used because it interferes with God's design for human sexuality.
. Ryder, " 'Natural Family Planning' Effective Birth Control Supported by the Catholic Church," British Medical Journal 307 (September 18, 1993), 723-725; P. Frank-Herrmann, et al., “The Effectiveness of a Fertility Awareness Based Method to Avoid Pregnancy in Relation to a Couple’s Sexual Behaviour During the Fertile Time: A Prospective Longitudinal Study,” Human Reproduction doi:10.1093/humrep/dem003 (February 2007):1–10.