* Malteser Arbeitsgruppe NFP, Natürlich und sicher. Das Praxisbuch, 2011, S. 29
** L.L. Sievert, C.A. DUBOIS, Validating signals of ovulation: Do women who think they know, really know?, American Journal of Human Biology, 17:310–320, 2005
* Malteser Arbeitsgruppe NFP, Natürlich und sicher. Das Praxisbuch, 2011, S. 29
** L.L. Sievert, C.A. DUBOIS, Validating signals of ovulation: Do women who think they know, really know?, American Journal of Human Biology, 17:310–320, 2005