1. Freundl G. Prospective European multi-center study of natural family planning (1989–1992): interim results. Adv Contracept. 1993;9:269–83.
2. De Leizaola MA. Première phase d'une étude prospective d'efficacité du planning familial naturel realisée en Belgique francophone. J Gynécol Obstet Biol Reprod. 1994;23:359–64.
3. Collins WP, Flynn AM et al. Temporal relationships between indices of the fertile period. Fertil Steril. 1983;39:647–55.
4. Von Fragstein M, Flynn AM, Royston P. Analysis of a representative sample of natural family planning users in England and Wales, 1984–1985. Int J Fertil. 1988;(suppl):70–7.
5. Rice RJ, Lanctot CA, Garcia-Devesa C. Effectiveness of the symptothermal method of natural family planning. An international study. Int J Fertil. 1981;26:222–30.