Ball M. A prospective field trial of the "ovulation method" of avoiding conception. Eur J Obstet Gynecol Reprod Biol. 1976, 6 (2): 63–6. PMID 985763.
Sinha G, Sinha A. A field trial of Billings' ovulation method for spacing and limitation of birth. J Indian Med Assoc. 1993, 91 (10): 255–6. PMID 8308307.
The National Women's Health Information Center. What is a typical menstrual period like?. U.S. Department of Health and Human Services. November 2002 [2005-06-11]. (原始内容存档于2015-06-26).
The National Women's Health Information Center. What is a typical menstrual period like?. U.S. Department of Health and Human Services. November 2002 [2005-06-11]. (原始内容存档于2015-06-26).