Good grief, klossg - at least some women have cervical mucus for 8 days - I do. I've read what I consider to be reliable and informed testimony from women who had intercourse 7 days before ovulation and conceived. Maybe some men have long-lived sperm and if they marry women with long CM phases, presto.
Wikipedia's "Menstrual Cycle" entry has footnotes to 3 scientific papers saying sperm can live 8 days.
If you want to make real sure NFP doesn't lead to pregnancy, you have to skip the pre-ovulatory time altogether.
And what do you do when your body starts throwing out a double ovulation now and then?
From Wikipedia, Menstrual Cycle:
"Sperm survive inside a woman for 3 days on average, with survival time up to five days considered normal. A pregnancy resulting from sperm life of eight days has been documented.[7][8][9]
# ^ Ball M (1976). "A prospective field trial of the "ovulation method" of avoiding conception". Eur J Obstet Gynecol Reprod Biol 6 (2): 63-6. PMID 985763.
# ^ Dr Evelyn Billings & Ann Westmore (2005). Trials of The Billings Ovulation Method. Retrieved on 2005-11-03.
# ^ Sinha G, Sinha A (1993). "A field trial of Billings' ovulation method for spacing and limitation of birth". J Indian Med Assoc 91 (10): 255-6. PMID 8308307. "
Mrs VS
"From Wikipedia, Menstrual Cycle:
"Sperm survive inside a woman for 3 days on average, with survival time up to five days considered normal. A pregnancy resulting from sperm life of eight days has been documented.[7][8][9]
# ^ Ball M (1976). "A prospective field trial of the "ovulation method" of avoiding conception". Eur J Obstet Gynecol Reprod Biol 6 (2): 63-6. PMID 985763.
# ^ Dr Evelyn Billings & Ann Westmore (2005). Trials of The Billings Ovulation Method. Retrieved on 2005-11-03.
# ^ Sinha G, Sinha A (1993). "A field trial of Billings' ovulation method for spacing and limitation of birth". J Indian Med Assoc 91 (10): 255-6. PMID 8308307. "
So I might not be stupid or crazy! What a relief.