What Is the Fertile Window? When You Can Get Pregnant

The fertile window is the short stretch of each cycle when intercourse can lead to pregnancy, and it lasts about six days. It runs from the five days before ovulation through to the day of ovulation itself. Sperm can survive for several days in fertile cervical mucus while an egg lives for only around a day after it is released, which is why the days leading up to ovulation are the ones that count.

When does the fertile window open and close?

The window opens as ovulation approaches and your body produces fertile cervical mucus, which helps sperm survive and travel. It closes once ovulation has passed and the egg is no longer available. In the landmark study by Wilcox and colleagues, published in the New England Journal of Medicine in 1995, conception only occurred when intercourse happened within a six-day window that ended on the day of ovulation. After ovulation, the chance of conceiving from that cycle drops away quickly.

How long does the fertile window last?

Around six days in total. That figure reflects two biological limits: sperm can live for up to about five days in the right cervical mucus, and the egg survives for roughly 24 hours after ovulation. Because sperm can wait for the egg but the egg cannot wait long for sperm, your most fertile days are the two or three days before ovulation and the day of ovulation itself.

How do you know when you are in your fertile window?

You can identify the window in real time by watching your body's fertility signs rather than relying on the calendar alone. The three main signs are cervical mucus, basal body temperature and, for some people, the luteinising hormone (LH) surge measured with ovulation tests.

Cervical mucus

As ovulation approaches, mucus usually becomes clearer, wetter and more stretchy, like raw egg white. This fertile-type mucus signals that the window is open. The Billings Ovulation Method and the Creighton Model are two structured approaches built around observing these changes.

Basal body temperature

Your resting temperature rises slightly, by around 0.3 degrees Celsius, just after ovulation and stays up until your next period. Temperature confirms that ovulation has happened, so it tells you the window is closing rather than opening.

Combining the signs

Tracking more than one sign is more reliable than tracking just one. The symptothermal method combines cervical mucus with basal body temperature, while the Marquette Method pairs mucus observations with an electronic hormone monitor. Using two signs lets one confirm the other.

Does the fertile window fall on the same days every cycle?

Not reliably. A later study by Wilcox and colleagues, published in the BMJ in 2000, found that the timing of the fertile window varies a great deal, even in women whose cycles are usually regular. In that study only about 30 percent of women had their fertile window entirely within days 10 to 17 of the cycle, the range often assumed by simple calendar rules. At least 10 percent of women were in their fertile window on any given day between days 6 and 21. This is why methods that track live fertility signs tend to be more dependable than counting calendar days alone.

Can you get pregnant outside the fertile window?

It is very unlikely to conceive from intercourse well before the window opens or after ovulation has clearly passed. The catch is that the window is not always where you expect it, and ovulation can happen earlier or later than usual. If you are avoiding pregnancy, the safest approach is to treat any fertile-type sign as a reason for caution and to confirm ovulation has passed before assuming the window has closed.

How Majella helps

Majella is a cycle and fertility tracking app made for women who practise Natural Family Planning. You record your daily signs, such as cervical mucus and basal body temperature, and Majella helps you see your fertile window as it unfolds rather than guessing from the calendar. Cycle tracking is free, the NFP features are a paid upgrade, and your data stays private on your own device.

Frequently asked questions

How many days a month can you get pregnant?

About six days in each cycle: the five days before ovulation and the day of ovulation. Your chance of conceiving is highest in the two to three days just before you ovulate.

Is the fertile window the same as ovulation?

No. Ovulation is the single day the egg is released, while the fertile window is the wider span of days around it when intercourse can lead to pregnancy. The window includes several days before ovulation because sperm can survive and wait for the egg.

Can you get pregnant right after your period?

It is possible, especially if you have shorter cycles or your period runs long, because the fertile window can open sooner than expected. Research shows fertile days are less predictable than many calendar rules assume, so tracking your signs is more reliable than counting days.

How accurate is tracking the fertile window?

When fertility signs are tracked correctly and consistently, a robust method like the symptothermal method is highly effective. A 2007 study in Human Reproduction reported around 0.4 to 0.6 unintended pregnancies per 100 women over 13 cycles with correct use, and about 1.8 per 100 across all users. Accuracy depends on learning the method well and applying it consistently.

References

Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. New England Journal of Medicine. 1995;333(23):1517-1521.

Wilcox AJ, Dunson D, Baird DD. The timing of the fertile window in the menstrual cycle: day specific estimates from a prospective study. BMJ. 2000;321(7271):1259-1262.

Frank-Herrmann P, Heil J, Gnoth C, et al. The effectiveness of a fertility awareness based method to avoid pregnancy in relation to a couple's sexual behaviour during the fertile time: a prospective longitudinal study. Human Reproduction. 2007;22(5):1310-1319.

Peragallo Urrutia R, Polis CB, Jensen ET, et al. Effectiveness of fertility awareness-based methods for pregnancy prevention: a systematic review. Obstetrics and Gynecology. 2018;132(3):591-604.

This article is for general education and is not medical advice. Speak with a qualified healthcare professional or a trained fertility awareness instructor for guidance about your own cycle and circumstances.

Related reading: How to Track Your Cycle with Basal Body Temperature and Cervical Mucus and How to Use Natural Family Planning to Get Pregnant.