Editing Calendar-based contraceptive methods (section)

===Failure rate===
One concern related to the use of calendar-based methods is their relatively high failure rate, compared to other methods of birth control. Even when used perfectly, calendar-based methods, especially the rhythm method, result in a high pregnancy rate among couples intending to avoid pregnancy. Of commonly known methods of birth control, only the [[cervical cap]] and [[contraceptive sponge]] have comparably high failure rates. This lower level of reliability of calendar-based methods is because their formulas make several assumptions that are not always true.<ref name="kippleycalendar">Kippley, p.154</ref>

The postovulatory ([[Menstrual cycle#Luteal phase|luteal]]) phase has a normal length of 12 to 16 days,<ref>Weschler, p.48.</ref> and the rhythm method formula assumes all women have luteal phase lengths within this range. However, many women have shorter luteal phases, and a few have longer luteal phases.<ref>Kippley, p.111</ref> For these women, the rhythm method formula incorrectly identifies a few fertile days as being in the infertile period.<ref name="kippleycalendar" /> Roughly 30-50% of women have phases outside this range.<ref>{{cite journal |last1=Marston |first1=Cicely A. |last2=Church |first2=Kathryn |title=Does the evidence support global promotion of the calendar-based Standard Days Method® of contraception? |url=https://pubmed.ncbi.nlm.nih.gov/26794286/ |journal=Contraception |access-date=14 October 2024 |pages=492–497 |doi=10.1016/j.contraception.2016.01.006 |date=June 2016|volume=93 |issue=6 |pmid=26794286 }}</ref>

Finally, calendar-based methods assume that all bleeding is true menstruation. However, mid-cycle or [[anovulatory bleeding]] can be caused by a number of factors.<ref>Kippley, pp.413-415</ref> Incorrectly identifying bleeding as menstruation will cause the method's calculations to be incorrect.<ref name="kippleycalendar" />