A person with a menstrual cycle lasting between 26 and 32 days will be most fertile between days 8 and 19 of their cycle. Ovulation, which is when the ovaries release an egg, is the most fertile period.

In theory, a person can become pregnant at any time in their cycle, but it is most likely to happen around ovulation. When ovulation happens will vary according to a person’s cycle.

The days during the menstrual cycle when a person is least likely to get pregnant are known as the ‘safe period.’

This article describes how to calculate the fertile window to aid or avoid conception.

A note about sex and gender

Sex and gender exist on spectrums. This article will use the terms “male,” “female,” or both to refer to sex assigned at birth. Click here to learn more.

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The average person’s menstrual cycle is between 28 and 32 days. Some people have shorter cycles, while others have much longer ones.

The first day of a person’s period is the first day of their menstrual cycle. Their period typically lasts 3 to 7 days.

According to The American College of Obstetricians and Gynecologists (ACOG), a person whose menstrual cycle is between 26 days and 32 days long will be most fertile between days 8 and 19 of their cycle.

Variations in the menstrual cycle usually happen in the follicular phase, which occurs before ovulation.

The luteal phase, which occurs from ovulation to the next period, is typically 14 days long.

Learn more about the phases of the menstrual cycle.

Ovulation occurs when one of the ovaries releases an egg. After release, the egg moves to the fallopian tube, where it will travel to the uterus over the course of 24 hours or so.

Pregnancy occurs if sperm travel to the fallopian tube and fertilizes the egg. If sperm does not fertilize the egg, the egg moves to the uterus and breaks down, ready to leave the body during the next menstrual period.

Learn more about sperm.

Calculating ovulation

According to the ACOG, ovulation occurs around 14 days before a person expects to have their next period if their monthly cycle is 28 days.

Most people ovulate between days 11 and 21 of their cycle. The first day of their last menstrual period (LMP) is day 1 of the cycle.

Ovulation does not always occur on the same day every month and can vary by a day or more on either side of the expected date.

Doctors call the part of the cycle around ovulation the fertile window because the chance of pregnancy is highest at this time. For example, if ovulation occurs on day 14, a person can conceive on that day or within the following 24 hours.

However, their fertile window begins a few days before ovulation because sperm can survive for up to 5 days inside the female body. So, even if a person does not have sex on day 14 or 15, it is still possible to become pregnant if they had sex without using contraception on days 9 to 13.

According to research from 2018, the likelihood of conception rises from day 8, reaching its maximum on day 13 and decreasing to zero by day 30.

However, it is essential to note that these findings should only act as a guideline. Every person and every cycle is different.

It can be helpful for a person to chart their monthly cycle and note the signs of ovulation to help pinpoint the exact day of ovulation each month.

Learn more about ovulation.

Signs of ovulation

Tracking the signs of ovulation can help someone determine the precise day they ovulate each month.

Signs include:

Some of these signs, such as basal body temperature, will continue to change after ovulation. For this reason, it is advisable not to use temperature to predict the fertile window.

It may be helpful for someone to track the signs over a few months to get an idea of what is typical for their body.

However, keep in mind that there are several variables, and the timing of ovulation can change month-to-month.

Another option is to use an ovulation predictor kit or fertility monitor.

Fertility aids measure the levels of specific hormones in the urine to determine the ovulation day each month. Some devices also identify days of peak fertility.

Using a combination of these methods may provide an individual with the best accuracy.

The following table, based on research from 2015, summarizes a typical menstrual cycle lasting 28 days and how fertile a person is likely to be at each stage.

Day of cycleStageFertility
1 to 7menstruationleast fertile stage
8 to 9post-menstruationpossible to conceive
10 to 14days around ovulationmost fertile
15 to 16post-ovulationpossible to conceive
17 to 28thickening of uterine liningless fertile — unlikely to conceive

To get a more specific range of highest fertility windows based on the day of ovulation, a person can track the first day of their last period in a pregnancy calculator.

Learn more about how to track ovulation.

To maximize the chances of becoming pregnant, a person could time sexual intercourse to occur during the 2 to 3 days leading up to and including ovulation.

Having sexual intercourse on any of these days may provide a 20% to 30% chance of pregnancy.

Other tips to improve the chances of conception include:

  • Have regular sexual intercourse: Pregnancy rates are highest among partners who have sex every 2 or 3 days throughout the month.
  • Avoid smoking: Smoking tobacco reduces fertility and impacts the health of a developing fetus.
  • Limit alcohol intake: Alcohol intake can reduce fertility in males and females and harm a fetus.
  • Maintain a moderate weight: People who are overweight or underweight are more likely to have irregular ovulation.
  • Reduce stress: While the research on whether anxiety or stress can reduce fertility is not conclusive, it is clear that reducing stress can provide some benefits for a person trying to conceive.
  • Manage comorbidities: Rule out or treat any medical causes that may contribute to infertility. In females, this can include hormonal problems, endometriosis, polycystic ovary syndrome (PCOS), uterine fibroids, autoimmune disorders, and more.

A doctor can assess a couple’s overall health and may be able to identify methods to improve the probability of conception.

Learn more about infertility in males and females.

What can affect male fertility?

Even if a person is having regular sexual intercourse and tracking ovulation, there can be reasons why the sperm cannot reach the egg.

Causes of male infertility include:

  • Sperm morphology: Differences in the size and shape of the sperm, also known as sperm morphology, can impact fertility.
  • Low sperm count: The male partner’s ejaculate fluid does not contain sufficient sperm. Fertility doctors consider a person to have a low sperm count if they have under 15 million sperm per milliliter of semen — the fewer the sperm, the lower the chances of conception.
  • Low sperm motility: Low sperm motility means the sperm cannot move as efficiently to reach the egg.

There are a variety of medical conditions that can cause any of these problems with male fertility, including:

  • hormonal imbalance
  • infection or injury to the testicles
  • surgery in the area
  • diseases, such as cancer

Learn more about sperm count.

Some people may wish to track their fertility to prevent pregnancy. This is known as the fertility awareness method.

The Centers for Disease Control and Prevention (CDC) charts fertility awareness-based methods of contraception as having a failure rate of up to 23% with typical use.

The chances of pregnancy are lowest during a person’s period and on the days on either side of the period.

However, they may still become pregnant if they have ovulated early or late in their cycle, as sperm can survive in the body for several days.

People wishing to use the fertility awareness method may want to speak with their doctor first.

Learn more about the types of contraception available.

Ovulation and the fertile window can change from cycle to cycle, but they may also alter with age. Fertility naturally begins to decline in females in their 30s. By age 40, a person’s chance of conceiving drops to 10% per cycle.

The number of eggs and egg quality decrease with age. Ovulation may also become irregular.

Some medical conditions, such as endometriosis or PCOS, also make conception more difficult.

Learn more about PCOS and fertility.

Birth control pills aim to prevent unintended pregnancy. The pill prevents pregnancy by releasing synthetic hormones that stop ovulation from occurring and the uterus lining from thickening.

So, even if the ovaries release an egg, a fertilized egg would be unable to implant in the wall of the uterus. The pill also thickens cervical mucus, making it harder for sperm to reach an egg.

According to the CDC, the typical use failure rate for the birth control pill is only 7%, making it a very effective birth control method.

Learn more about the pill and its potential side effects.

Females under 35 years of age with no apparent health or fertility problems can try conceiving for one year before contacting a doctor about fertility.

Those over 35 years of age could seek medical advice after 6 months of trying to conceive.

A female with any of the following who wishes to become pregnant could also contact a doctor about fertility:

Females who track their fertile window with the aim of getting pregnant could see their doctor for preconception planning.

As well as identifying barriers to conceiving, a doctor can advise on the use of folic acid or prenatal supplements to encourage a safe pregnancy.

Males who wish to conceive could contact a doctor about fertility if they have any of the following:

Here are the answers to some frequently asked questions about when pregnancy can occur.

Can I get pregnant 2 days before my period?

Days 17 to 28 before the first day of a person’s period are the days when the uterine lining begins to thicken. While a person is less likely to conceive during this time, conception is still possible.

Learn more about the uterine lining.

Can I get pregnant on my period?

Although pregnancy is still possible, a female is less fertile while on their period.

Learn more about getting pregnant during a period.

How many days after your period can you get pregnant?

A female can get pregnant at any time during her menstrual cycle. The likelihood of pregnancy is highest during ovulation, which typically occurs on days 10 to 14.

Intermenstrual bleeding can occur between periods. To pinpoint the start of a menstrual cycle, a female must correctly identify their actual period.

Learn more about bleeding between periods.

Each person’s menstrual cycle is different. On average, it lasts 28 to 32 days. A person is most fertile during ovulation, which occurs around days 10 to 14.

A person can calculate the exact days of ovulation each month by counting from the first day of their period.

However, certain medical conditions may affect ovulation or impact the sperm’s ability to reach the egg.

People who have been unable to conceive for over a year could talk with a fertility specialist.

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