The rhythm or calendar method is a natural approach to birth control. It requires a person to monitor their menstrual cycle and signals from the body to estimate when sex is least likely to result in pregnancy.

The rhythm method is a type of natural family planning or fertility awareness. It is not the most reliable way of preventing pregnancy.

With typical use, the traditional rhythm method, which only involves counting days on a calendar, has a failure rate of 8–25%. With perfect use, the failure rate is less than 5%. Forms of natural family planning that require a person to monitor body signals, such as those that use basal body temperature, tend to be more effective.

In this article, we will look at what the rhythm method is, how effective it is, its advantages and disadvantages, and how to use it.

“Perfect” vs. typical birth control use

This article talks about perfect and typical use of birth control. Perfect use describes how effective a form of birth control is if people use it exactly as the instructions recommend every they have sex. Typical use describes how effective a form of birth control is if a person sometimes uses it per the instructions but may also use it irregularly or imperfectly.

Even with perfect use, contraception is not 100% effective. People should discuss birth control options with a healthcare professional to find the right option for them.

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The rhythm method is a natural form of birth control. It relies on a person tracking their fertility throughout the menstrual cycle, ensuring they do not have sex on days when they are most fertile. Alternatively, people may use backup forms of contraception on these days.

The rhythm method requires a person to know when they are ovulating. Ovulation is the point in the menstrual cycle when the ovaries release an egg. Tracking this is essential to the rhythm method, as sperm can fertilize an egg at any time during the fertile window. This is the period shortly before, during, and after ovulation.

Dr. Leo J. Latz first popularized the rhythm method in the early 20th century in the United States. His approach assumed that all or most menstrual cycles were 28 days in duration, with ovulation occurring around day 14.

This is not accurate. A 2020 study found that just 12.4% of respondents had a 28-day menstrual cycle, while 87% had cycles ranging from 23–35 days. Even among people with a 28-day cycle, the typical ovulation range varied by up to 10 days.

For this reason, newer forms of the rhythm method have emerged, such as natural family planning or fertility awareness. Often, these newer techniques involve monitoring a range of fertility signs to predict the fertile window rather than counting days on a calendar.

Some signs a person might track include:

  • their average date of ovulation
  • ovulation tests, which measure luteinizing hormone
  • cervical fluid, which changes consistency when a person approaches ovulation
  • cervical position and texture
  • basal body temperature, which is a person’s body temperature as soon as they wake up, and which goes up after ovulation

The effectiveness of the rhythm method depends on several factors, such as:

  • how regular a person’s cycle is
  • how reliably they can predict ovulation
  • the tools they use to predict it

The traditional rhythm method has a high failure rate, at 8–25% with typical use. This means that, in everyday circumstances, 8–25 people in every 100 who use this method will become pregnant.

The failure rate with perfect use is better, at 5%. But this still means that 5 in every 100 people who use it will become pregnant. By comparison, intrauterine devices have a failure rate of 0.1 to 0.4%.

Other forms of natural family planning have the following levels of effectiveness:

  • Billings ovulation method: This method involves monitoring the cervical mucus for signs of ovulation. The failure rate with perfect use is 3%, but 3–22% with typical use.
  • 2-day method: This is a variation on the Billings ovulation method and requires a person to monitor their cervical mucus twice per day. The failure rate with perfect use is 4%, and up to 14% with typical use.
  • Sympto-thermal method: This method combines measuring basal body temperature every day with tracking cervical mucus. The failure rate is only 0.4% with perfect use, but 2–33% with typical use.
  • Sympto-hormonal method: This method combines body temperature, cervical mucus, and hormone monitoring. The failure rate is 1–2% with perfect use and 11–14% with typical use.

The rhythm method and other types of fertility awareness have several advantages compared with other types of birth control. They:

  • cause no side effects
  • do not involve taking medications
  • require no doctor appointments or prescriptions
  • can be inexpensive and accessible

The rhythm method may also:

  • increase awareness of one’s menstrual cycle
  • become a useful tool for getting pregnant, if a person does decide to become a parent
  • help identify a lack of ovulation, which can be a sign of underdiagnosed conditions, such as polycystic ovary syndrome

These methods are also suitable for people who have religious objections to birth control.

The main disadvantage of the rhythm method is that it is not always reliable, especially for people who have variable menstrual cycles or ovulation days.

This may not be a significant drawback for people who would not necessarily mind if they became pregnant or who would prefer to avoid beginning new medications because they want to get pregnant in the near future.

But those who definitely do not want to become pregnant or need protection from sexually transmitted infections (STIs) may want to use another method of birth control. Alternatively, they could combine the rhythm method with barrier protection, such as condoms.

Other potential disadvantages include:

  • Effort: The rhythm method and other fertility awareness techniques require effort and commitment to monitor the body and figure out when ovulation is happening. If a person forgets, they could get pregnant.
  • Regularity: People with unpredictable or irregular lifestyles may find it difficult to stick to the rhythm method. For example, people who repeatedly wake up at night or get up at different times in the morning may not be able to get an accurate body temperature measurement.
  • Knowledge: Effectively monitoring fertility requires knowledge and practice. While many people can learn how to do it, it is a new skill for many beginners. Using backup contraception can protect against any slipups.
  • Responsibility: This approach to birth control places all of the responsibility on a person who menstruates, whereas, with other options, partners who do not menstruate can take on or share responsibility, if applicable.
  • Cost: The newest and most effective types of natural family planning often involve equipment to track body temperature, hormones, and ovulation. This can require thermometers, ovulation tests, apps, and more.

The rhythm method may be a good option for a person if they:

  • can tolerate the risk of pregnancy, or are willing to use a backup method of contraception
  • have regular menstrual cycles
  • have the time, knowledge, and commitment to track their cycle
  • do not need to use barrier methods to prevent STIs

If both partners have undergone STI screenings, and neither has other sexual partners, STI protection might not be necessary.

The original rhythm method uses a simple but potentially unreliable technique to calculate when it is safest to have sex. It involves counting days on a calendar to figure out when ovulation will happen.

In the past, people thought this would be around day 14 of a person’s menstrual cycle. A person is less likely to get pregnant after ovulation and in the first week of their cycle, which is during their period.

A person who has regular cycles and ovulation days may still be able to use this approach, but generally, it is better to track body signals. Some options for this include:

  • Ovulation testing: To determine when it is unsafe to have intercourse, people can test for ovulation daily in the first half of the cycle. When a person gets a positive test, they are more likely to be fertile then and for a few days afterward.
  • Basal body temperature: Basal body temperature charting can detect ovulation after it has happened. A person takes their temperature at the same time each morning, then looks for a slight but sustained increase after ovulation. By the time this increase is apparent, a person is no longer fertile.
  • Cervical fluid: The cervical fluid method looks for changes in the fluid that comes from the cervix and out of the vagina. Early in the cycle, a person may notice very little fluid or thick fluid. As they become more fertile, the fluid may become very watery or develop the consistency of egg whites. There may also be more of it.

The safest time to have sex is when a person has no signs of fertility and is several days past ovulation.

Many birth control apps use a form of the rhythm method. However, birth control apps have modern technology that can collate and analyze information from a person’s previous cycles. The creators of these apps argue this makes them more effective at calculating safe days than humans.

Birth control apps vary slightly in their approach. Some only gather data using an algorithm or artificial intelligence to predict the fertile window. Some also track current fertility signs, such as body temperature.

The company Natural Cycles submitted data to the Food and Drug Administration (FDA) that suggests that their app may be more effective than natural family planning without an app.

It followed 15,570 app users aged 18–45 from September 1, 2017, to April 30, 2018. According to the data, the failure rate from using the Natural Cycles app was 1% with perfect use and 6.5% with typical use.

Birth control apps may suit people who would prefer not to try and calculate their own fertile window, but it may be useful to speak with a gynecologist about this first to see if it is a good option.

The rhythm method is a form of birth control that relies on a person tracking ovulation to determine which days they will be the least fertile. Originally, the method involved counting days on a calendar to figure out when ovulation would happen, but additional measures now exist.

A person may combine calendar tracking with cervical mucus, body temperature, or ovulation testing. They may also combine these methods with backup contraception, such as condoms, which offer additional protection and can prevent STI transmission.

The rhythm method and natural family planning generally are not completely reliable, even with perfect use. If a person has questions about whether this option is right for them, a sexual health clinic or gynecologist can offer advice.