Calendar-based contraceptive methods

It is not known if historical cultures were aware of what part of the menstrual cycle is most fertile. In the year 388, Augustine of Hippo wrote of periodic abstinence. Addressing followers of Manichaeism, his former religion, he said, "Is it not you who used to counsel us to observe as much as possible the time when a woman, after her purification, is most likely to conceive, and to abstain from cohabitation at that time...?"[8] If the Manichaieans practiced something like the Jewish observances of menstruation, then the "time... after her purification" would have indeed been when "a woman... is most likely to conceive."[9] Over a century previously, however, the influential Greek physician Soranus had written that "the time directly before and after menstruation" was the most fertile part of a woman's cycle; this inaccuracy was repeated in the 6th century by the Byzantine physician Aëtius. Similarly, a Chinese sex manual written close to the year 600 stated that only the first five days following menstruation were fertile.[9] Some historians believe that Augustine, too, incorrectly identified the days immediately after menstruation as the time of highest fertility.[10]

Written references to a "safe period" do not appear again for over a thousand years.[9] Scientific advances prompted a number of secular thinkers to advocate periodic abstinence to avoid pregnancy:[11] in the 1840s it was discovered that many animals ovulate during estrus. Because some animals (such as dogs) have a bloody discharge during estrus, it was assumed that menstruation was the corresponding most fertile time for women. This inaccurate theory was popularized by physicians Bischoff, Félix Archimède Pouchet, and Adam Raciborski.[9][10] In 1854, an English physician named George Drysdale correctly taught his patients that the days near menstruation are the least fertile, but this remained the minority view for the remainder of the 19th century.[9]

In 1905 Theodoor Hendrik van de Velde, a Dutch gynecologist, showed that women only ovulate once per menstrual cycle.[12] In the 1920s, Kyusaku Ogino, a Japanese gynecologist, and Hermann Knaus, from Austria, working independently, each made the discovery that ovulation occurs about fourteen days before the next menstrual period.[13] Ogino used his discovery to develop a formula for use in aiding infertile women to time intercourse to achieve pregnancy.

In 1930, Johannes Smulders, a Roman Catholic physician from the Netherlands, used Knaus and Ogino's discoveries to create a method for avoiding pregnancy. Smulders published his work with the Dutch Roman Catholic medical association, and this was the official rhythm method promoted over the next several decades.[13] In 1932 a Catholic physician, Dr. Leo J Latz, published a book titled The Rhythm of Sterility and Fertility in Women describing the method,[11] and the 1930s also saw the first U.S. Rhythm Clinic (founded by John Rock) to teach the method to Catholic couples.[14]

In the first half of the 20th century, most users of the rhythm method were Catholic; they were following their church's teaching that all other methods of birth control were sinful. In 1968 the encyclical Humanae vitae included the statement, "It is supremely desirable... that medical science should by the study of natural rhythms succeed in determining a sufficiently secure basis for the chaste limitation of offspring." This is interpreted as favoring the then-new, more reliable symptoms-based fertility awareness methods over the rhythm method. Currently, many fertility awareness teachers consider the rhythm method to have been obsolete for at least 20 years.[4]

New attention was drawn to calendar-based methods in 2002, when the Institute for Reproductive Health at Georgetown University introduced the Standard Days Method. Designed to be simpler to teach and use than the older rhythm method, the Standard Days Method is being successfully integrated into family planning programs worldwide.

To find the estimated length of the pre-ovulatory infertile phase, eighteen (18) is subtracted from the length of the woman's shortest cycle. To find the estimated start of the post-ovulatory infertile phase, eleven (11) is subtracted from the length of the woman's longest cycle.[16] A woman whose menstrual cycles ranged in length from 30 to 36 days would be estimated to be infertile for the first 11 days of her cycle (30-19=11), to be fertile on days 12–25, and to resume infertility on day 26 (36-10=26). When used to avoid pregnancy, such fertility awareness-based methods have a typical-use failure rate of 25% per year.[15]

A CycleBeads birth control chain, used for a rough estimate of fertility based on days since menstruation

Developed by Georgetown University's Institute for Reproductive Health, the Standard Days Method has a simpler rule set and is more effective than the rhythm method.[15][17] A product called CycleBeads was developed alongside the method to help the user keep track of estimated high and low fertility points during her menstrual cycle. The Standard Days Method may only be used by women whose cycles are usually between 26 and 32 days in length; it is estimated that between 50% and 60% of women of reproductive age satisfy this condition.[18] In this system:

• Days 1–7 of a woman's menstrual cycle are considered infertile
• Days 8–19 are considered fertile; considered unsafe for unprotected intercourse
• Day 20 through the end of the cycle are considered infertile.

When used to avoid pregnancy, the Standard Days Method has been estimated[19] to have perfect-use efficacy of 95% and typical-use efficacy of 88%.[17][18] These figures are based on a 2002 study in Bolivia, Peru, and the Philippines of women of reproductive age having menstrual cycles between 26 and 32 days,[17][20]^: 505 and on a 2014 study in Turkey.[21] However, other researchers have criticized the methodology of the first study, have stated that the 95% figure has been presented to the public in misleading ways, and have argued that the true efficacy figures are likely to be much lower.[22]

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.[16]

The postovulatory (luteal) phase has a normal length of 12 to 16 days,[27] 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.[28] For these women, the rhythm method formula incorrectly identifies a few fertile days as being in the infertile period.[16]

Calendar-based methods use records of past menstrual cycles to predict the length of future cycles. However, the length of the pre-ovulatory phase can vary significantly, depending on the woman's typical cycle length, stress factors, medication, illness, menopause, breastfeeding, and whether she is just coming off hormonal contraception. If a woman with previously regular cycles has a delayed ovulation due to one of these factors, she will still be fertile when the method tells her she is in the post-ovulatory infertile phase. If she has an unusually early ovulation, calendar-based methods will indicate she is still in the pre-ovulatory infertile phase when she has actually become fertile.[16]

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.[29] Incorrectly identifying bleeding as menstruation will cause the method's calculations to be incorrect.[16]

It has been suggested that pregnancies resulting from method failures of periodic abstinence methods are at increased risk of miscarriage and birth defects due to aged gametes at the time of conception.[30] Other research suggests that timing of conception has no effect on miscarriage rates,[31] low birth weight, or preterm delivery.[32]

Luc Bovens has suggested that unprotected intercourse in the infertile periods of the menstrual cycle may still result in conceptions, but create zygotes incapable of implanting.[33] Bovens maintains that, if one defines abortion to include any destruction of fertilized eggs, then the use of the rhythm method probably results in a large number of abortions.