Calculating Your Fertile Window: How Long Does Ovulation Last?

by Mar 16, 2021

If you are trying to get pregnant, calculating your fertile window is everything. Having sex during your most fertile days can significantly increase your chances of conceiving

If you are trying to get pregnant, calculating your fertile window is everything. Having sex during your most fertile days can significantly increase your chances of conceiving.

You have probably heard that the time surrounding ovulation is the best time to have sex for optimal fertility when trying to conceive (TTC). But how long does ovulation last? And how do you use this information to discover your estimated fertile window?

In this blog post, we’ll help you calculate your estimated fertile window so you can prepare in advance for your most fertile time of the month while TTC, offering you the highest chance possible of getting pregnant.

How Long Does Ovulation Last?

The process surrounding ovulation lasts 24 to 36 hours during each menstrual cycle. Eggs develop inside of follicles, small sacs inside the ovaries. Follicle-stimulating hormone, or FSH, stimulates these follicles to grow. The growing follicles also produce estrogen.

As estrogen reaches its upper threshold, luteinizing hormone, or LH, peaks. When luteinizing hormone surges (also called the LH surge), it causes a egg to exit the follicle and travel down the fallopian tube. What we call “ovulation” is the rupturing of the follicle and release of the egg to the fallopian tube.


After its release, the egg has between 12 and 24 hours to be fertilized by sperm in the fallopian tube. If it is not fertilized, it disintegrates and is shed alongside the uterine lining during your menstrual period. However, if the sperm fertilized the egg, it may result in pregnancy.

Understanding Your Ovulation Cycle

Your fertile window consists of the five days leading up to ovulation and the day of ovulation itself. You can determine your fertile window by testing your fertility hormones with Mira.

After ovulation, an egg only has 12 to 24 hours to be fertilized by sperm in the fallopian tube. Fortunately for those trying to conceive, however, your fertile window lasts longer than that. Semen can survive in the reproductive tract for up to five days. Therefore, intercourse during the five days leading to ovulation also has a high likelihood of resulting in pregnancy. If you don’t believe us, check out this study tracking 221 women planning for pregnancy, and their 996 menstrual cycles.

The calendar method tracks ovulation by asking you to note your normal menstrual cycle (21 to 35 days) for six cycles. Your fertile window is calculated by subtracting 18 from the shortest cycle to find the first day of your fertile window, and subtracting 11 from the longest cycle to find the last day of your fertile window.

However, using this standard calculation to determine your chances of getting pregnant isn’t completely reliable. Ovulation can occur during a large range of days, from day six to 21. Even among women with the same menstrual cycle length, studies have found differences in the date of ovulation.

Regardless of when you ovulate, your sex hormones follow a predictable path before ovulation. Estrogen usually rises in the three to five days leading to ovulation. Luteinizing hormone (LH) also surges 24 to 36  hours before ovulation, peaking 10 to 12 hours prior to ovulation.

The changes in these two hormones cause ovulation. Therefore, your hormones are one of the most accurate and fundamental signs of ovulation to track. You are usually fertile when your estrogen levels begin to rise and reach your peak fertility day at the LH surge. Ovulation predictor kits (OPKs) can measure this surge in LH.

Your chances of conceiving usually peak on the day right before ovulation. Blood work and ultrasounds can detect ovulation but won’t track the whole five-day fertile window. Here’s what research based on several studies combined, says about the chance of conceiving related to ovulation:

  • Day 5 Before Ovulation: 0.4-7% chance of conception
  • Day 4 Before Ovulation: 8-17%
  • Day 3 Before Ovulation: 8-23%
  • Day 2 Before Ovulation: 13-29%
  • Day 1 Before Ovulation: 21-34%
  • Day of Ovulation: 8-33%
  • Day 1 After Ovulation: 0.8-11%

Signs of Ovulation

You can increase your chances of getting pregnant by noticing the changes in your body that signify you are about to ovulate. These signs of ovulation can signify when you are entering your fertile window and should prepare to have intercourse in order to conceive.

Changes in Cervical Mucus

Throughout most of your menstrual cycle, your cervical mucus is thick, creamy, and white or off-white. When you are ovulating, your cervical mucus will become clear and slippery, matching the consistency of an egg white. Using a tracking tool like the Mira App’s advanced calendar can help you get in touch with the consistency of your cervical mucus to better recognize when ovulation is about to begin.

Changes in Basal Body Temperature (BBT)

Your body temperature dips a bit just before your ovary releases an egg. Then, 24 hours after the egg’s release, your temperature rises and stays up for several days. Before ovulation, a woman’s BBT averages between 97°F (36.1°C) and 97.5°F (36.4°C). After ovulation, it rises to 97.6°F (36.4°C) to 98.6°F (37°C). Recording your temperature every day with a basal thermometer can help you recognize the patterns in your body temperature before your next ovulation.

Changes in the Position of the Cervix

When you are ovulating, your cervix may become higher, softer, and more open to prepare for accepting sperm in order to become pregnant. If you feel comfortable, you can use a finger to gently feel for changes in the cervix prior to ovulation. The Mira App’s advanced calendar can help you keep track of the changes you feel surrounding ovulation to help you better predict when you are ovulating

Changes in Urine Hormone Concentration

The most reliable way to determine when you are ovulating is to measure the changes in your hormones directly. Mira’s digital technology allows you to observe changes in your sex hormones throughout the menstrual cycle, all from the comfort of your home. 24 to 36  hours prior to ovulation, your Mira App will show a surge in LH to signify that your body is about to release an egg. This surge will peak 10 to 12 hours prior to ovulation.

If you aren’t a Mira user yet, here’s a bit about how our tracking tool works: by immersing the wand in a urine sample for 10 seconds and inserting the wand into the Mira digital analyzer, you can receive targeted insights into your fertile window after as little as one cycle of tracking with Mira.

Signs Ovulation is Over

After ovulation, your fertility returns to baseline and you are less likely to conceive. If you are trying to time intercourse with your fertile window in order to get pregnant, it’s essential to understand both the signs that you are about to ovulate and the signs that ovulation is over.

Changes in Cervical Mucus

During ovulation, your cervical mucus takes on a slippery, egg-white consistency. After ovulation, you will see your cervical mucus return to its normal consistency: a thick, creamy, white or off-white discharge. Tracking these changes in the Mira App can help you learn to identify the end of your fertile window before your next menstrual cycle.

Changes in Basal Body Temperature (BBT)

When you ovulate, your BBT may increase by less than 0.5 of a degree Fahrenheit. Ovulation has likely occurred when the slightly higher temperature remains steady for three days or more. Your BBT remains higher for the second half of your menstrual cycle. Taking your temperature each morning before you get out of bed with a basal thermometer, and recording the results in the Mira App, can give you more clues that your fertile window has ended.

Changes in the Position of the Cervix

The cervix is high, soft, and open during ovulation so it can accept sperm in order to help you become pregnant. After ovulation, you will feel the cervix return to its original position: low, hard, and closed. During this period, the cervix will feel a bit like the tip of your nose to the touch. You can gently feel for changes in your cervix to help you determine if your fertile window has ended, and record the details in the Mira App for future reference.

Changes in Urine Hormone Concentration

After ovulation, the Mira App will show your urine hormone concentration levels returning to basal. However, that does not mean you should stop tracking with the Mira digital analyzer. You should track your urine hormone concentration 10 to 20 times throughout each menstrual cycle. Remember: the more data you can collect during your menstrual cycle, the more accurately Mira can predict when you are about to ovulate during your next cycle.

What is a Fertile Window?

Your fertile window refers to the six most fertile days of your menstrual cycle. These are the rare days that you are likely to get pregnant from sexual intercourse. Your chances of conceiving outside the fertile window are not zero, but they are very small. So, if you really want to get pregnant, you won’t want to miss your fertile window!

The fertile window describes the days leading up to ovulation as well as the day after ovulation. Usually, it means the four to five days leading to ovulation, until 24 hours after ovulation. Ovulation happens when the egg is released from the ovary and travels down the fallopian tube. If sperm travels into the fallopian tube, the egg may be fertilized and you may become pregnant.

Your eggs have a short life span. They only live for up to 24 hours after they are released from the ovary. Therefore, sperm must meet the egg before degradation happens. That is why there is usually only one fertile day after ovulation.

The good news is that sperm can live up to five days in a woman’s cervical mucus, provided the mucus is of good quality. This is why the fertile window includes the days prior to ovulation, as well as ovulation itself.

Calculating Your Fertile Window: When Are You Most Fertile?

You are not equally likely to conceive on every day of your fertile window. Your most fertile days are the day leading to ovulation and the day of ovulation. These two days are called your peak fertility.

LH often surges during these two days. OPKs can measure this hormone surge. However, since menstrual cycles vary from month to month and from woman to woman, sometimes an OPK gives a false-positive or false-negative result. If you notice your cycles vary greatly, you may want to see a fertility specialist, as irregular cycles may sometimes indicate reproductive issues.

Yourchance of conception is low anytime beyond 24 hours after ovulation. 24 hours after ovulation, the egg degrades, and there is no chance of conception. After that, your fertility level gradually increases as you approach the day of your next ovulation.

The Calendar Method

Calculating your fertile window by counting backwards from your shortest and longest cycle is called the calendar method. However, even for women with regular cycles, the calendar method may not be accurate. We may not always have the same cycle length, and we may not always ovulate 14 days before our next period. It is no surprise, then, that the calendar method usually takes six months or more of continuous logging to get an accurate prediction.

Basal Body Temperature (BBT)

The BBT method looks for a 0.5 F temperature increase after ovulation. Although this methodology sounds convenient, the egg only lives up to 24 hours after ovulation. You may miss the majority of the fertility window by testing BBT alone to determine when you are most fertile.

Cervical Mucus

Your cervical mucus changes in texture throughout the menstrual cycle. Around the time of ovulation, it becomes clear and stretchy, resembling an egg white. Tracking your cervical mucus is a great way to pay more attention to your body. It is natural and non-invasive. However, everybody’s cervical mucus is different, and it may take three to six months of tracking to fully understand the unique changes in your cervical mucus that signify you are about to ovulate.

Hormone Testing

Hormone testing is one of the most scientifically trusted ways to detect a fertile window. Testing your levels of estrogen and LH with Mira’s digital fertility analyzer can help you predict your full fertile window, not just the date of ovulation. To know exactly what’s happening with your body, Mira measures the numeric levels of fertility hormones with a urine test, just like bringing a lab test to your home. Mira tracks your fertility hormones continuously, and the Mira App tells you what to do to either try for or avoid having a baby.

✔️ Medically Reviewed by Banafsheh Kashani, MD, FACOG

Banafsheh Kashani, MD, FACOGBanafsheh Kashani, M.D., FACOG is a board-certified OB/GYN and specialist in reproductive endocrinology and infertility at Eden Fertility Centers, and has been treatingcouples and individuals with infertility since 2014.

Dr. Kashani has conducted extensive research in female reproduction, with a specific focus on the endometrium and implantation.

Additionally, Dr. Kashani has authored papers in the areas of fertility preservation, and fertility in women with PCOS and Turners syndrome. She also was part of a large SART-CORS study evaluating the trend in frozen embryo transfers and success rates.

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