Your Fertile Window: Which Days You Can Get Pregnant

Your fertile window is the short stretch of each cycle when sex can lead to pregnancy. It lasts about six days, it ends on the day you ovulate, and it moves around more than most calendar apps assume. Here is what the research shows about when it opens, how many days after your period it usually starts, and how to find yours in practice.

What is the fertile window?

The fertile window is the six days that end on the day of ovulation: the five days before the egg is released, plus ovulation day itself.¹ The definition comes from a study in which 221 women trying to conceive collected daily urine samples across 625 cycles. Every one of the 192 pregnancies started from sex inside that six-day stretch.¹ ASRM uses the same six-day definition when counseling people trying to conceive.²

The window is six days long because of a mismatch in lifespans. Sperm can survive in the reproductive tract for several days, while the egg survives only about 12 to 24 hours after it is released.⁸ Sex a few days before ovulation can still lead to pregnancy because sperm are already waiting. Sex after the egg is gone usually cannot.

Which days are the most fertile?

The two days before ovulation and ovulation day itself give the best odds. In the 1995 study, the chance of conception rose from about 10% for sex five days before ovulation to about 33% for sex on ovulation day.¹ A later reanalysis that corrected for errors in pinpointing ovulation found the single best day was the day before ovulation, with odds falling close to zero once ovulation had passed.³ ASRM summarizes it the same way: peak fertility comes from sex within two days before ovulation.²

That is why the most useful thing a test can do is warn you a day or two ahead. A sign that only confirms ovulation afterwards, such as a temperature rise, tells you the best days have already gone.

How many days after your period do you ovulate?

Count from the first day of your period, not from the day it ends. The standard calendar estimate assumes the second half of the cycle lasts about 14 days, so ovulation is placed 14 days before the next period is due: day 14 in a 28-day cycle, day 16 in a 30-day cycle, and so on.² If your period lasts five days, a 28-day cycle puts estimated ovulation about nine days after bleeding stops.

Treat this table as a starting point. Real ovulation days spread out widely. Among 949 volunteers who tested their urine daily, the most likely ovulation day in a 28-day cycle was day 16, and even that day held only 21% of ovulations.⁶ Data from nearly 76,000 cycles logged with a connected ovulation test showed a 10-day spread of ovulation days in 28-day cycles, with day 15 the most common.⁷ For a personal estimate, enter your own dates in the calculator below. It uses the same count as the table, and the window it shows also includes the day after ovulation as a buffer.

Why your fertile window moves

Most of the difference between short and long cycles comes from the first half of the cycle, the time it takes a follicle to mature. In an analysis of 612,613 cycles, the follicular phase averaged 16.9 days but ranged from 10 to 30 days, while the luteal phase after ovulation averaged 12.4 days.⁵ Only 13% of cycles in that dataset were exactly 28 days long.⁵

The practical effect is large. In the 2000 follow-up of the original study, only about 30% of women had their fertile window entirely within cycle days 10 to 17, the days many guidelines used to quote.⁴ On every day from day 6 to day 21, at least 10% of women were in their fertile window.⁴ An estimated 2% were already fertile by day 4, and 17% by day 7.⁴ Even women with regular cycles had a 1% to 6% chance of being fertile on the day their next period was due.⁴ If you are wondering whether sex near your period counts, see can you only get pregnant during ovulation?

How to track ovulation

The best approach combines a cheap early warning with something that confirms ovulation happened. Here is what each method can and cannot tell you.

• Calendar and apps. Free and instant, but they assume your cycle behaves like an average one. In the 949-volunteer study, apps predicted the actual ovulation day no better than 21% of the time.⁶ Use a calendar estimate to decide when to start testing, not when to stop trying.
• LH tests. Urine strips detect the luteinizing hormone surge that comes shortly before ovulation. A Cochrane review found that timing sex with urine ovulation tests probably increases the chance of a live birth compared with not predicting ovulation (risk ratio 1.36, from one trial of 844 women, moderate-quality evidence).⁹ See how long after an LH surge you ovulate.
• Cervical mucus. Mucus becomes clear and slippery in the days before ovulation and is free to check.¹⁰ Our cervical mucus guide covers what each stage means.
• Basal body temperature. Temperature rises slightly after ovulation and stays up until your period, so it confirms ovulation rather than predicting it.¹⁰ It is most useful for learning your pattern over several cycles.
• Fertility monitors. Devices that read several hormones can show the window opening earlier than LH alone and confirm ovulation afterwards.

What to do once you know your window

You do not need to hit a single perfect day. ASRM advises sex every one to two days during the fertile window, and notes that sex two to three times a week across the cycle gives nearly the same results.² It also cautions that tracking should guide frequent sex, not replace it, because rigid schedules can add stress.² Our guide to the best time to have sex to conceive turns this into a simple plan.

When to get help

ACOG recommends an infertility evaluation after 12 months of trying if you are under 35, after six months if you are 35 or older, and a conversation with your doctor right away if you are over 40.¹¹ Very irregular or absent periods are a reason to see a clinician sooner, because they can mean you are not ovulating regularly. For more on the step you are in, go back to the Try hub.