http://www.billings-centre.ab.ca/
http://www.familyplanning.net/
http://www.mlink.net/~serena/eng/start.html   (sinto-térmico)
http://www.fertilityuk.org/nfps781.html (sinto-térmico)
http://www.usc.edu/hsc/info/newman/resources/nfp.html (os vários métodos naturais)
http://www.priestsforlife.org/index.html (site católico)