1. Symptoms and hormonal changes accompanying ovulation;Billings;Lancet,1972
2. Cervical mucus and identification of the fertile phase of the menstrual cycle;Flynn;Br J Obstet Gynaecol,1976
3. Natural family planning. I. The peak symptom and estimated time of ovulation;Hilgers;Obstet Gynecol,1978
4. A trial of the ovulation method of family planning in Tonga;Weissmann;Lancet,1974
5. Atlas of the Ovulation Method;Billings,1974