Wondering how the 99.6% effectiveness data for the sympto-thermal method of NFP was derived? Here are a few research studies for your reading pleasure, spanning across 4 decades:
Natural Family Planning Method As Effective As Contraceptive Pill
The article:
European Society for Human Reproduction and Embryology. “Natural Family Planning Method As Effective As Contraceptive Pill, New Research Finds.” ScienceDaily. ScienceDaily, 21 February 2007. www.sciencedaily.com/releases/2007/02/070221065200.htm
and a link to read the actual study itself:
P. Frank-Herrmann, J. Heil, C. Gnoth, E. Toledo, S. Baur, C. Pyper, E. Jenetzky, T. Strowitzki, G. Freundl; “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, Volume 22, Issue 5, 1 May 2007, Pages 1310- 1319, https://doi.org/10.1093/humrep/dem003
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“…During ‘perfect use’ the pregnancy rate at 12 months is 0.63%. When only protected intercourse takes place in the fertile phase the pregnancy rate is 0.45%. The symptothermal method of NFP is most unforgiving for imperfect use (unprotected intercourse in the fertile phase). However, it is extremely effective when either abstinence or protected intercourse is used in the fertile phase.“
Frank-Herrmann P1, Freundl G, Gnoth C, Godehardt E, Kunert J, Baur S, Sottong U; “Natural family planning with and without barrier method use in the fertile phase: efficacy in relation to sexual behavior: a German prospective long-term study.” Advances in Contraception, 1997 Jun-Sep; 13 (2-3):179-89, https://www.ncbi.nlm.nih.gov/pubmed/9288336
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Table IV: Overall Use-Effectiveness Using Post- and Pre-Menstrual Days shows Pearl Index = 0.8, based on 12 surprise pregnancies in 17,026 cycles among 491 fertile women
J. Roetzer; “The Sympto-Thermal Method: Ten Years of Change” Linacre Quarterly, Volume 45, Number 4, Article 8; November 1978; https://epublications.marquette.edu/lnq/vol45/iss4/8
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From Guttmacher Institute: see the “Contraceptive Effectiveness” table
https://www.guttmacher.org/fact-sheet/contraceptive-use-united-states
Table Source: Hatcher RA et al., eds., Contraceptive Technology, 20th ed., New York: Ardent Media, 2011.