Understanding which days of the menstrual cycle a person is most fertile 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 trying to get pregnant can try to track their menstrual cycle and plan 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.
Although menstrual cycles can vary from person to person, the average length is
The ovaries release an egg, or ovulate, about 10–16 days before menstruation begins. The time around ovulation is called the fertility window because it is the most fertile days in the menstrual cycle.
It is possible to get pregnant during the 5 days before ovulation and on the same day. An egg can survive up to 24 hours after release, and sperm can survive up to 7 days after sex. Therefore, it is also possible to get pregnant in the 2 days after ovulation, but it is less likely.
As a result, there are approximately 21 days in the menstrual cycle when 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 can ovulate within days of their period ending.
Menstrual cycles can shorten with age, especially after the age of
Fertility peaks 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 within the 1 or 2 days after a period, as 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 get 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. This usually happens 10-16 days before the start of the next period, although menstrual cycles can vary in length.
For fertilization to occur, a sperm must fertilize the egg. Once the ovaries release the egg, it enters the fallopian tube. If sex has taken place, the egg will meet the sperm.
The likelihood of getting pregnant increases if sperm is already in the fallopian tube when ovulation occurs. For this reason, people are most likely to get pregnant in the days before ovulation.
Once the egg is fertilized, it moves further down the fallopian tube to the uterus, where it implants in the uterine lining and develops into an embryo.
However, if a sperm does not fertilize the egg, the uterine lining breaks down and leaves the body during menstruation.
Fertility awareness methods (FAMs) are ways for people to track their monthly menstrual cycles and identify their least fertile days.
People who do not want to get pregnant can then plan to have sex on those days. They may also want to avoid 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.
Track their temperature
Tracking temperature throughout the menstrual cycle can help predict ovulation. Body temperature usually rises around ovulation, from 96-98ºF (35-36ºC) to 97-99ºF (36-37ºC).
People using 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.
Examine 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 smooth, 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.
Just like with body temperature, an individual can use a chart or phone app to track their cervical mucus.
Use 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 track their menstrual cycle for at least six periods before using the calendar method as a form of birth control.
This method may be more difficult to use for people with different menstrual cycle lengths and people with menstrual cycles shorter than 27 days.
Several free and paid tools and apps are available to help people track their cervical mucus and temperature over their menstrual cycle. Planned Parenthood also offers a calendar card 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 about 76–88% effective. These numbers mean that 12–24 couples out of 100 using FAMs can 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:
- take advice from a healthcare professional who has expert knowledge of the use of FAMs
- to check 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
- use of hormonal contraceptives in the months before using FAMs
It is also important that people check their temperature at the same time every day. If a person forgets or records their temperature at another 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.
FAMs also offer no protection against STIs.
The only completely reliable method of preventing 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:
More permanent measures, such as sterilization and vasectomy, are 99% effective.
People who worry that their FAM has not been reliable can find reassurance in using emergency contraception.
There are two main types of emergency contraception: IUDs and emergency contraceptive pills (ECPs).
These are small, T-shaped devices made of copper and plastic. As a form of emergency contraception, they release copper to prevent the fertilized egg from implanting.
A doctor can place this device in the uterus within 5 days of unprotected sex or up to 5 days after ovulation.
IUDs are 99% effective at preventing pregnancy. A doctor must place these devices in a clinic.
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 can rise and fall over a person’s menstrual cycle. It increases around the time of ovulation.
People who want to reduce their likelihood of getting pregnant may want 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, it is still not the most reliable method of contraception.