Understanding which days of the menstrual cycle a person is most fertile on can help people avoid pregnancy.

A person’s likelihood of pregnancy can increase or decrease depending on where they are in their menstrual cycle.

People who are trying to avoid conceiving can try tracking their menstrual cycle and planning to have sex around their most fertile days.

This article looks at how fertility rises and falls during a person’s menstrual cycle and some signals to look out for to help determine a low fertility day.

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Although menstrual cycles can vary from person to person, the average length is 28 days.

The ovaries release an egg, or ovulate, around 10–16 days before menstruation begins. The time around ovulation is called the fertility window because these are the most fertile days in the menstrual cycle.

It is possible to become pregnant during the 5 days leading up to ovulation and on the same day. An egg can survive for up to 24 hours after release, and sperm can survive for up to 7 days after sex. Therefore, it is also possible to get pregnant in the 2 days after ovulation, but this is less likely.

As a result, there are around 21 days in the menstrual cycle on which pregnancy is less likely. The days before and during menstruation are the least fertile days of the menstrual cycle.

People with a menstrual cycle that is shorter than 28 days could ovulate within days of their period ending.

Menstrual cycles may shorten with age, particularly after the age of 35 years.

Fertility increases sharply around 12–14 days before menstruation, so unprotected sex is more likely to result in pregnancy during that time.

It is unlikely but possible that conception will occur in the 1 or 2 days following a period since sperm can survive for up to 7 days after sex.

For example, a couple may have sex the day after a period ends. If ovulation occurs on that day or within the next 6 days, pregnancy is possible.

People with shorter menstrual cycles are more likely to become pregnant in the days after a period ends because they ovulate earlier.

Ovulation is the process of an egg developing within the ovaries and the ovaries releasing the egg each month. It usually happens 10–16 days before the start of the next period, though menstrual cycles can vary in length.

For conception to occur, a sperm must fertilize the egg. Once the ovaries release the egg, it passes into the fallopian tube. If sex has occurred, the egg will meet the sperm.

The likelihood of becoming pregnant increases if sperm are already in the fallopian tube when ovulation occurs. For this reason, people are most likely to become pregnant in the days leading up to ovulation.

Once fertilized, the egg travels farther down the fallopian tube to the uterus, where it implants in the uterine lining and develops into an embryo.

If a sperm does not fertilize the egg, however, the uterine lining breaks down and leaves the body during menstruation.

Fertility awareness methods (FAMs) are ways that people can track their monthly menstrual cycles and identify their least fertile days.

People who do not wish to become pregnant can then plan to have sex on those days. They may also wish to avoid having vaginal sex or use birth control on the days they identify as particularly fertile.

FAMs are also sometimes known as natural family planning or the rhythm method.

There are different types of FAMs that people can use to identify their most and least fertile days. Some people use just one, while others use a combination of the following.

Tracking their temperature

Tracking temperature throughout the menstrual cycle can help predict ovulation. Body temperature typically rises around ovulation, from 96–98ºF (35–36ºC) to 97–99ºF (36–37ºC).

People who use temperature as a FAM will need to take their temperature every day and track it with a chart or fertility app. Experts recommend monitoring temperature for 3 months before using it as a reliable FAM.

Examining their cervical mucus

Cervical mucus, or vaginal discharge, changes in color, texture, and volume throughout the menstrual cycle.

On a person’s most fertile days, the mucus will feel slippery, similar to raw egg white. On their less fertile days, it will be sticky and cloudy. On their least fertile days, there may be little or no mucus at all.

Just as with body temperature, an individual can use a chart or phone app to track their cervical mucus.

Using a calendar

The calendar method involves tracking the menstrual cycle and manually working out which days are most and least fertile.

Planned Parenthood suggests that people follow their menstrual cycle for at least six periods before using the calendar method as a form of birth control.

This method may be harder to use for people with varying menstrual cycle lengths and people with menstrual cycles that are shorter than 27 days.

Several free and paid-for tools and apps are available to help people track their cervical mucus and temperature across their menstrual cycle. Planned Parenthood also offers a calendar chart to print and update each day.

Some people may prefer to use a string of beads, called Cyclebeads, to help them keep track of their menstrual cycle.

According to the National Health Service (NHS), FAMs can be up to 99% effective if people follow them carefully.

However, Planned Parenthood estimates that FAMs are around 76–88% effective. These numbers mean that 12–24 couples out of 100 who use FAMs may still conceive.

Other forms of contraception, such as intrauterine devices (IUDs) and implants, are typically more effective than FAMs.

It is possible to increase the effectiveness of FAMs by:

  • taking advice from a healthcare professional who has expert knowledge of using FAMs
  • checking one’s body temperature, cervical mucus, menstrual cycle length, and other fertility signs every day
  • avoiding vaginal sex or using another form of birth control on one’s most fertile days

Certain factors can affect fertility signals, such as:

  • illness, stress, and travel in different time zones
  • vaginal infections, such as thrush or a sexually transmitted infection (STI)
  • excessive alcohol consumption
  • irregular periods
  • using hormonal contraception in the months before using FAMs

It is also important that people check their temperature at the same time each day. If a person forgets or records their temperature at a different time of day, their readings will not be accurate.

The NHS says that it can take 3–6 months to become familiar with one’s menstrual cycle, so FAMs are not an easy, quick, or completely reliable way to prevent pregnancy.

Also, FAMs do not offer any protection against STIs.

The only completely reliable method to prevent pregnancy is abstinence from vaginal sex. However, other forms of birth control are highly effective when people use them correctly.

With typical use, the levels of effectiveness for each form of birth control are as follows:

Form of birth controlEffectiveness
birth control implant99%
IUD99%
birth control shot94%
birth control vaginal ring91%
contraceptive patch91%
birth control pills91%
diaphragm88%
birth control sponge76–88%
spermicide and contraceptive gelup to 86%
cervical capup to 86%
condom85%
internal condom79%
withdrawal before ejaculationup to 78%

More permanent measures, such as sterilization and vasectomy, are 99% effective.

People who have concerns that their FAM has not been reliable may find reassurance in using emergency contraception.

There are two main types of emergency contraception: IUDs and emergency contraceptive pills (ECPs).

IUDs

These are small, T-shaped devices made of copper and plastic. As a form of emergency contraception, they release copper to stop the fertilized egg from implanting.

A doctor can insert this device into the uterus within 5 days of unprotected sex or up to 5 days after ovulation.

IUDs are 99% effective in preventing pregnancy. A doctor must place these devices in a clinic.

ECPs

A person should take an ECP as soon as possible and within 5 days of having unprotected sex, depending on the type of pill.

ECPs are available for purchase at drugstores without restriction.

Fertility may rise and fall across a person’s menstrual cycle. It increases around the time of ovulation.

People who wish to reduce their likelihood of becoming pregnant may wish to use FAMs to track their fertility and schedule sex.

For FAMs to be effective, it is essential for a person to track several months of fertility signals to chart high fertility and low fertility days. However, this is still not the most reliable method of contraception.