There are two errors in the letter by Evans on the effectiveness of methods of natural family planning (NFP).1 Both the failure rates during typical use and the failure rates during perfect use are too low. The failure rates for the pill, male condom, and spermicides cited by Evans are based on the National Surveys of Family Growth (NSFG) in the United States, whereas the rates he cites for methods of NFP are from clinical studies. Estimates of contraceptive failure based on the NSFG for spermicides, fertility awareness based (FAB) methods, diaphragm, male condom, oral contraceptive pills, and Depo-Provera are generally much higher than estimates based on clinical studies/trials.2 The vast majority of FAB users rely on calendar rhythm (87% in the 1995, 78% in the 2002, and 83% in the 2006-2008 NSFG).3 Separate estimates of contraceptive failure during typical use of other FAB methods cannot be obtained because the sample sizes are too small. Correctly computed failure rates during perfect use are available for only four NPF methods: Standard Days (5%), TwoDay (4%), Ovulation (3%) and Symptothermal (0.4%).

1. Evans C. Natural family planning is effective and culturally acceptable. BMJ 2012;345:e4908
2. Trussell J. Contraceptive efficacy. In: Hatcher RA, Trussell J, Nelson AL, Cates W, Kowal D, editors. Contraceptive Technology: Twentieth Revised Edition. New York: Ardent Media, 2011.
3. Mosher WD, Jones J. Use of contraception in the United States: 1982-2008. National Center for Health Statistics. Vital Health Stat 2010; 23(29). Table 4. Available from: http://www.cdc.gov/NCHS/data/series/sr_23/sr23_029.pdf. Accessed 24 July 2012.

Competing interests: None declared