Rhythm method

# Overview
The rhythm method, also known as the calendar method or the Knaus-Ogino method (named after Hermann Knaus and Kyusaku Ogino), is a method of natural birth control that involves counting days of a woman's menstrual cycle in order to achieve or avoid pregnancy.  A recently developed variant of the rhythm method is known as the Standard Days Method.

# Incorrect use of the term
The phrase "the rhythm method" is frequently used, in error, to describe the behavior of any people who have unprotected sex yet wish to avoid pregnancy.  While the failure rate of rhythm is very high compared to most other methods of birth control, correct use of the rhythm method prevents a significant number of pregnancies.  Statements that calendar-based methods are "not methods of birth control" or "the same as nothing at all" are false.
The phrase "the rhythm method" may also be incorrectly used to describe the practice of observational fertility awareness (FA) methods. This often comes as a source of chagrin to practitioners of FA, because FA methods are significantly more effective than rhythm.  Many FA teachers consider the rhythm method to have been obsolete for at least 20 years.
For several decades, especially in the early twentieth century, the rhythm method was promoted by members of the Catholic Church as the only morally acceptable form of family planning.  Methods accepted by this church are referred to as natural family planning (NFP) - so at one time, the rhythm method was synonymous with NFP.  However, the modern Catholic Church also accepts certain uses of FA, and some sources now use the terms "fertility awareness" and "natural family planning" interchangeably.  This overlap between uses of the terms "the rhythm method", NFP, and FA may contribute to confusion.

# Description and effectiveness
The following describes use and effectiveness of the method for the purpose of avoiding pregnancy.
Most menstrual cycles have several days at the beginning that are infertile (pre-ovulatory infertility), a period of fertility, and then several days just before the next menstruation that are infertile (post-ovulatory infertility).  The first day of red bleeding is considered day one of the menstrual cycle.  The formula for the rhythm method requires the woman to know the length of her menstrual cycles.

## Knaus-Ogino Method
To find the estimated length of the pre-ovulatory infertile phase, nineteen (19) is subtracted from the length of the woman's shortest cycle.  To find the estimated start of the post-ovulatory infertile phase, ten (10) is subtracted from the length of the woman's longest cycle.
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, the rhythm method has a perfect-use failure rate of up to 9% per year.

## Standard Days Method
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. 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 always between 26 and 32 days in length.  In this system, days 1-7 of a woman's menstrual cycle are considered infertile.  Days 8-19 are considered fertile.  Infertility is considered to resume beginning on day 20.  When used to avoid pregnancy, the Standard Days Method has a perfect-use failure rate of 5% per year.

## Perimon
A software program developed in Germany from 1995 to 2001, Perimon is a stricter variant of the Knaus-Ogino method.  It requires a greater period where unprotected sex is not allowed when used to avoid pregnancy, designating a maximum of 10.5 days each cycle as infertile.  No clinical studies have been done to determine effectiveness, but the program's developers claim a perfect-use failure rate of 4% per year.  The Perimon software requires a paid subscription.

## Imperfect use
Imperfect use of the rhythm method would consist of not correctly tracking the length of the woman's cycles, thus using the wrong numbers in the formula, or of having unprotected intercourse on an identified fertile day.  The discipline required to keep accurate records of menstrual cycles, and to abstain from unprotected intercourse, makes imperfect use fairly common.  The actual failure rate of the rhythm method is 25% per year.

# Reasons for high failure rate
Even when used perfectly, the rhythm method results 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 the rhythm method is because its formula makes several assumptions that are not always true.
The postovulatory (luteal) phase has a normal length of 12 to 16 days, 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. For these women, the rhythm method formula incorrectly identifies a few fertile days as being in the infertile period.
The rhythm method uses 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 rhythm method tells her she is in the post-ovulatory infertile phase.  If she has an unusually early ovulation, the rhythm method will indicate she is still in the pre-ovulatory infertile phase when she has actually become fertile.
Finally, the rhythm method assumes that all bleeding is true menstruation.  However, mid-cycle or anovulatory bleeding can be caused by a number of factors.  Incorrectly identifying bleeding as menstruation will cause the rhythm method's calculations to be incorrect.

# Current utilization of Standard Days method
The Standard Days method (SDM) is increasingly being introduced as part of family planning programs in developing countries.  The method is satisfactory for many women and men who find other methods unacceptable; offering it through family planning centers results in a significant increase in contraceptive use among couples who do not want to become pregnant.  The low cost of the method may also enable it to have a significant positive impact in countries that lack funding to provide other methods of birth control.

# History
It is not known exactly when it was first discovered that women have predictable periods of fertility and infertility.  St. Augustine wrote in the year 388 of periodic abstinence practiced by the Manichaeans, "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...?"  One book states that " had been recommended... by a few secular thinkers since the mid-nineteenth century".  The Catholic Church first recorded official statement on periodic abstinence to avoid pregnancy is from 1853.
However, how this method prevented pregnancy was not understood until the early twentieth century.  In 1905 Theodoor Hendrik Van de Velde, a Dutch gynecologist, showed that women only ovulate once per menstrual cycle.  In the 1920s, Kyusaku Ogino, a Japanese gynecologist, and Hermann Knaus, from Austria, independently discovered that ovulation occurs about fourteen days before the next menstrual period.  Ogino used his discovery to develop a formula for use in aiding infertile women time intercourse to achieve pregnancy. In 1930, John Smulders, Roman Catholic physician from the Netherlands, used this discovery 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.
Early Catholic doctrine considered complete sexual abstinence to be the most holy state for humans, with marriage being a holy state for those without the fortitude required by an abstinent life. The Manichaeans (the group the early Church father St. Augustine wrote of) believed that it was immoral to create any children, thus (by their belief system), trapping souls in mortal bodies. Augustine condemned them for their use of periodic abstinence: "From this it follows that you consider marriage is not to procreate children, but to satiate lust."  In more recent times, an 1853 ruling of the Catholic Church's Sacred Penitentiary addressed the topic of periodic abstinence to avoid pregnancy.  Distributed to confessors, the ruling stated that couples who had, on their own, begun the practicing of periodic abstinenceespecially if they had "legitimate reasons"were not sinning by doing so. In 1880, the Sacred Penitentiary reaffirmed the 1853 ruling, and went slightly further.  It suggested that, in cases where the couple was already practicing artificial birth control, and could not be dissuaded to cease attempting birth regulation, the confessor might morally teach them of periodic abstinence.  However, into the early twentieth century, it was believed by some Catholics that the only licit reason for sexual intercourse was an attempt to create children.
While maintaining procreation as the primary function of intercourse, the December 1930 encyclical Casti Connubii by Pope Pius XI gave the highest form of recognition to a secondaryunitivepurpose of sexual intercourse.  This encyclical stated that there was no moral stain associated with having marital intercourse at times when "new life cannot be brought forth."  Although this referred primarily to conditions such as current pregnancy and menopause, the Sacred Penitentiary in yet another ruling in 1932, and the majority of Catholic theologians also interpreted it to allow moral usefor couples with "upright motives"of the newly created Rhythm Method. In 1932 a Catholic physician published a book titled The Rhythm of Sterility and Fertility in Women describing the method, and the 1930s also saw the first U.S. Rhythm Clinic (founded by John Rock) to teach the method to Catholic couples.  A minority of Catholic theologians, however, continued to doubt the morality of periodic abstinence, and some historians consider two speeches delivered by Pope Pius XII in 1951 to be the first unequivocal acceptance of periodic abstinence by the Catholic Church.
Humanae Vitae, published in 1968 by Pope Paul VI, addressed a pastoral directive to scientists: "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 fertility awareness methods over the Rhythm Method.
It has been suggested that unprotected intercourse in the infertile periods of the menstrual cycle may still result in conceptions, but create embryos incapable of implanting.
It has also 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. The most recent research, however, suggests timing of conception has no effect on miscarriage rates, low birth weight, or preterm delivery.