It is already clearly stated in the Talmud tractate Niddah, that a woman only becomes pregnant in specific periods in the month, which seemingly refers to ovulation.

In 1905 Theodoor Hendrik van de Velde, a Dutch gynecologist showed that women only ovulate once per menstrual cycle.

[10] 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.

[11] Ogino used his discovery to develop a formula for use in aiding infertile women to time intercourse to achieve pregnancy.

In 1930, John Smulders, a 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 first formalized system for periodic abstinence: the rhythm method.

[11] In the 1930s, Reverend Wilhelm Hillebrand, a Catholic priest in Germany, developed a system for avoiding pregnancy based on basal body temperature.

Two speeches delivered by Pope Pius XII in 1951 gave the highest form of recognition to the Catholic Church's approval—for couples who needed to avoid pregnancy—of these systems.

[9][13] In the early 1950s, John Billings discovered the relationship between cervical mucus and fertility while working for the Melbourne Catholic Family Welfare Bureau.

John and Sheila Kippley, lay Catholics, joined with Dr. Konald Prem in teaching an observational method that relied on all three signs: temperature, mucus, and cervical position.

[18] Toni Weschler started teaching in 1982 and published the bestselling book Taking Charge of Your Fertility in 1995.

[3] Femtech companies such as Dot[23] and Natural Cycles have also produced new studies and apps to help women avoid pregnancy.

This usually refers to a temperature reading collected when a person first wakes up in the morning (or after their longest sleep period of the day).

The production of fertile cervical mucus is caused by estrogen, the same hormone that prepares a woman's body for ovulation.

When a woman is in an infertile phase of her cycle, the cervix will be low in the vaginal canal; it will feel firm to the touch (like the tip of a person's nose); and the os—the opening in the cervix—will be relatively small, or "closed".

As a woman becomes more fertile, the cervix will rise higher in the vaginal canal, it will become softer to the touch (more like a person's lips), and the os will become more open.

Mucus- and temperature-based methods used to determine post-ovulatory infertility, when used to avoid conception, result in very low perfect-use pregnancy rates.

[28] The use of certain calendar rules to determine the length of the pre-ovulatory infertile phase allows unprotected intercourse during the first few days of the menstrual cycle while maintaining a very low risk of pregnancy.

Keeping a basal body temperature chart enables accurate identification of menstruation, when pre-ovulatory calendar rules may be reliably applied.

[35] Some couples prefer this method because the monitor reading is objective and is not affected by sleep quality as basal body temperature can be.

Fertility awareness has a number of unique characteristics: By restricting unprotected sexual intercourse to the infertile portion of the menstrual cycle, a woman and her partner can prevent pregnancy.

During the fertile portion of the menstrual cycle, the couple may use barrier contraception or abstain from sexual intercourse.

Records of basal body temperatures, especially, but also of cervical mucus and position, can be used to accurately determine if a woman is ovulating, and if the length of the post-ovulatory (luteal) phase of her menstrual cycle is sufficient to sustain a pregnancy.

Fertile cervical mucus is important in creating an environment that allows sperm to pass through the cervix and into the fallopian tubes where they wait for ovulation.

Fertility charts can help diagnose hostile cervical mucus, a common cause of infertility.

If this condition is diagnosed, some sources suggest taking guaifenesin in the few days before ovulation to thin out the mucus.

Knowing an estimated date of ovulation can prevent a woman from getting false negative results due to testing too early.