Editing Calendar-based contraceptive methods (section)

==Potential concerns==

===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" />

===Embryonic health===
It has been suggested that pregnancies resulting from failures of periodic abstinence methods are at increased risk of miscarriage and birth defects due to aged gametes at the time of conception.<ref>{{cite journal | author=Gray, RH | title=Aged gametes, adverse pregnancy outcomes and natural family planning. An epidemiologic review |journal=Contraception |date=October 1984 |volume=30 |issue=4 |pages=297–309 |pmid=6509983 |doi=10.1016/S0010-7824(84)80022-0 }}</ref> Other research suggests that timing of conception has no effect on miscarriage rates,<ref>{{cite journal |vauthors=Gray RH, Simpson JL, Kambic RT | title=Timing of conception and the risk of spontaneous abortion among pregnancies occurring during the use of natural family planning | journal=American Journal of Obstetrics and Gynecology |date=May 1995 | volume=172 | issue=5 | pages=1567–1572 | pmid=7755073 | doi=10.1016/0002-9378(95)90498-0 }}</ref> low birth weight, or preterm delivery.<ref>{{cite journal |vauthors=Barbato M, Bitto A, Gray RH |title=Effects of timing of conception on birth weight and preterm delivery of natural family planning users |journal=Advances in Contraception |date=June–September 1997 |volume=13 |issue=2–3 |pages=215–228 |pmid=9288339 |doi=10.1023/A:1006508106197 |s2cid=24939823 |display-authors=etal}}</ref>

===Destruction of fertilized eggs===
Philosopher Luc Bovens has suggested that the use of the rhythm method probably results in a large number of abortions, because unprotected intercourse in the infertile periods of the menstrual cycle may still result in conceptions but create zygotes incapable of implanting.<ref>{{cite journal |author=Luc Bovens |title=The rhythm method and embryonic death |journal=Journal of Medical Ethics |year=2006 |volume=32 |pages=355–356 |url= |doi=10.1136/jme.2005.013920 |pmid=16731736 |pmc=2563373 |issue=6}}</ref>  Bovens's argument requires the assumption that any and all destruction of fertilized eggs is abortion.