Understanding Your Fertile Window: When Are You Most Likely to Get Pregnant?

Maybe you’re hoping to be pregnant, or you’re exploring options to try to avoid it. You might just want to understand what’s happening with your body or your partner’s body. Regardless of your fertility plans, it’s helpful to know what’s happening in each stage of the menstrual cycle, including when a pregnancy is most likely to occur. 

What is a menstrual cycle?

About once a month, starting at puberty, most people with ovaries and a uterus will menstruate, or have a period. The time between periods varies among people. It can even change over your lifetime. But a normal menstrual cycle is anywhere from 21 to 35 days. 

Menstruation is just one part of the menstrual cycle. In the weeks before your period, hormones stimulate the ovaries to release an egg. This is called “ovulation.” The egg then passes into the fallopian tube. If the egg meets up with sperm and is fertilized, it may implant in the lining of the uterus, and a pregnancy begins. If the egg isn’t fertilized, hormone levels fall and the lining of the uterus is shed. This results in a menstrual period. 

What are the most fertile days of a menstrual cycle? 

There are just a few days in the menstrual cycle when pregnancy can happen. That’s because an egg can only be fertilized for about 12 to 24 hours after ovulation. But sperm can live in the uterus for up to 5 days. So having sex on the day of ovulation — or during the 5 days before ovulation — might result in pregnancy. A pregnancy is most likely if you have sex in the 3 days before ovulation or on the day of ovulation. This is called your fertile window.

Estimating ovulation: What is the calendar method?

So how do you know when ovulation is happening? Everyone’s cycle is different, so to estimate the day of ovulation, begin by counting the days of the menstrual cycle. The first day of menstrual bleeding is called “Day 1.” Count each day until you start your next period — and the count starts over again. 

Once you’ve tracked your periods for a few months, you can figure out how long your average menstrual cycle is. Ovulation usually happens about 14 days before the start of the next period.

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Let’s look at some examples:

• If you have a period every 28 days, the day of ovulation is around Day 14. (Remember that the first day of the period is Day 1.) You’re most likely to become pregnant if you have sex on Days 12, 13, or 14.

• If you have a period every 35 days, ovulation happens around Day 21. You’re most fertile on Days 19, 20, and 21.

Menstrual cycle length will vary from person to person. But cycles can also change throughout your lifetime. They can be shorter, longer, or irregular when they first start in adolescence or when it’s time for menopause. Other factors — like stress, nutrition, exercise, and certain health conditions — can also cause changes in the menstrual cycle.

Other ways to help predict when you’re the most fertile

Even people who have regular menstrual cycles may ovulate earlier or later than the expected 14 days before the next period. Here are other ways to figure out if you’re ovulating:

• Check your temperature. Following your basal body temperature involves checking your temperature every morning. A rise of 0.4°F (0.2°C) or more above your usual temperature usually means that you’ve ovulated. 

• Check your cervical mucus. Cervical mucus changes throughout the menstrual cycle. You might notice more clear, whitish, stretchy mucus just before you ovulate.  

• Use a test kit. Ovulation predictor kits usually come with five to seven test sticks. These detect a rise in luteinizing hormone (LH) just before you ovulate. You can test your urine in the morning for several days in a row, starting a few days before your calculated ovulation date. 

When in the menstrual cycle are you less likely to get pregnant?

As we’ve discussed, you can only get pregnant around the time of ovulation. The trick is figuring out when that’s happening. It’s unlikely to get pregnant outside of the fertile window if you’ve tracked your cycles using the calendar or one of the other methods — and you’re pretty confident you know when you’re ovulating. 

Are you fertile during or right after your period?

In general, it’s least likely to become pregnant during your period. It may be possible to get pregnant in the few days after your period, depending on the length of your menstrual cycle. Remember, your fertile window will typically be based on how many days you have between periods. But this becomes more difficult to predict if you have irregular cycles. That being said, it may still be possible to get pregnant right after your period if you have a very short time between periods. 

How effective is natural family planning to prevent pregnancy?

Just as you can track your cycle and time sex to become pregnant, you can also use it to try to avoid pregnancy. But how effective is it as a contraceptive method?

Here are some statistics on the effectiveness of natural family planning as a contraceptive method:

• With perfect use (you have regular cycles and never have sex during the fertile window), 1 to 5 pregnancies will occur for every 100 females using the method.

• With typical use (your cycles aren’t always regular, or you and your partner sometimes have sex in the fertile window), 12 to 24 pregnancies will occur for every 100 females using the method.

To compare this level of effectiveness, let’s look at the number of pregnancies that occur with typical use of other contraceptive methods:

• Levonorgestrel intrauterine device (IUD): Less than 1 per 100

• Copper IUD: Less than 1 per 100

• Hormone implant: Less than 1 per 100

• Hormone shot: 4 per 100

• Birth control pills: 7 per 100

• Birth control patch or vaginal ring: 7 per 100

• Male condom: 13 per 100

• Female condom: 21 per 100

So if you’re trying to avoid getting pregnant, there are more reliable ways to prevent pregnancy. But no method is 100% effective if you’re having sex.

What other factors can affect fertility?

There are other conditions that can impact fertility. Common factors include:

• Older age (female fertility begins to decline in the 30s and male fertility in the 40s)

• Smoking

• Heavy alcohol use

• Marijuana (male fertility)

• Hormone imbalances

• Blocked tubes in reproductive organs

• Stress

• Chemical exposures 

Can irregular menstrual cycles affect fertility?

Using the menstrual cycle to predict when you’re fertile is less reliable if your cycles are less than 26 days, greater than 32 days, or irregular.

It’s not uncommon to have periods that don’t come exactly on time. But it’s important to get  medical attention if:

• You bleed between periods.

• You bleed after having sex.

• Your periods last more than 7 days, or you soak through a tampon or pad every hour.

• Your periods happen less than 21 days apart.

• Your periods happen more than 35 days apart.

• Your periods always vary by more than 1 week.

• You haven’t had a period for more than 3 months.

• You start to bleed again after menopause.

It’s also a good idea to talk with a healthcare professional about contraceptive options if you don’t want to be pregnant. And having a checkup helps if you’re planning a pregnancy. Schedule a visit if you’re concerned about infertility. Most experts recommend testing if you don’t achieve pregnancy after 1 year of unprotected sex — or after 6 months if you’re older than 35 years.

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Written by:

Anne Jacobson, MD, MPH

Anne Jacobson, MD, MPH has been a board-certified family physician since 1999. She worked as a full-scope family physician (inpatient, outpatient, obstetrics, and office procedures) in the Cook County Ambulatory Health Network for 15 years.

Edited by:

Karla Robinson, MD

Karla Robinson, MD, is a medical editor for GoodRx. She is a licensed, board-certified family physician with almost 20 years of experience in health through varied clinical, administrative, and educational roles.

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