US Catholic Conference of Bishops: Natural Family Planning Office: http://www.usccb.org/issues-and-action/marriage-and-family/natural-family-planning/

Couple to Couple League: the couple to couple league offers a wealth of support and resources dedicated to promoting Natural Family Planning. https://ccli.org/

Humanae Vitae: a letter from Pope Paul VI on contraception http://www.vatican.va/holy_father/paul_vi/encyclicals/documents/hf_p-vi_enc_25071968_humanae-vitae_en.html

Natural Family Planning FAQs: an article from www.foryourmarriage.org, a great resource for all the stages of married life sponsored by the United States Conference of Catholic bishops. . http://www.foryourmarriage.org/family-planning/