Find your fertile window – cycle, body signs and tests

Short answer: The chance of becoming pregnant is greatest during the approximately six days ending on the day of ovulation, and highest the day before ovulation. Ovulation usually occurs around 14 days before your next period. Because cycles vary, the period is difficult to pinpoint exactly. Having sexual intercourse a couple of times a week covers the fertile period without you having to count.

What is meant by the fertile period?

In research, the fertile period is usually described as the five days before ovulation and the day of ovulation itself. This is possible because sperm can remain alive when the egg is released. The exact day of ovulation is difficult to determine at home, so calendar days should be seen as estimates.

Research that has prospectively followed women with hormone measurements points in the same direction: almost all pregnancies can be linked to sexual intercourse during a six-day period ending on the day of ovulation, with the highest probability on the day before ovulation. After ovulation, the probability drops quickly.

How to calculate approximately when you ovulate

A menstrual cycle is calculated from the first day of menstruation to the first day of the next menstruation and is usually 21–35 days long. Ovulation occurs approximately two weeks before your next period, which for many—but far from everyone—falls in the middle of the cycle.

The simplest method is to count 14 days backward from the day you expect your period. The fertile period then becomes approximately the five days before the estimated ovulation day plus the day of ovulation. The method assumes that you have fairly regular cycles and that you can predict when your period will come—which makes it an estimate, not an exact date.

It is also worth knowing that the often-cited rule of thumb about days 10–17 of the cycle is less accurate than many think. In the prospective study published in the BMJ, a significant proportion of women had their fertile period outside that interval, and on virtually every day between day 6 and day 21, there was a substantial probability of being fertile. The evidence therefore suggests that variation between—and within—individuals is large.

Signs and tests that can help you

Some notice their ovulation, others not at all. Common signs highlighted by 1177 are that cervical mucus becomes clearer and stickier and can resemble raw egg white, mild aching in the lower abdomen, increased sexual desire, and tender breasts. The signs are useful indicators but are not reliable on their own.

Ovulation tests from pharmacies measure the LH surge in urine and can indicate that ovulation is expected within approximately one day. Basal body temperature measurement can show that ovulation has already occurred, because the temperature rises slightly afterward—which makes the method more useful for seeing patterns over several cycles than for planning individual days. Menstrual cycle apps are usually based on statistical models of previous cycles and are therefore no more accurate than the data you enter.

Does timing sexual intercourse actually help?

The evidence here is more nuanced than one might think. A Cochrane review from 2023, which included seven randomized studies involving a total of approximately 2,464 women or couples, found that timed sexual intercourse using urine-based ovulation tests likely increased the chance of pregnancy and live birth in women under 40 who had been trying for less than 12 months, compared to sexual intercourse without ovulation prediction.

At the same time, the authors point out that several of the studies were funded by manufacturers of ovulation tests, and that the results should therefore be interpreted with caution. For other methods—for example, fertility awareness-based methods—the evidence was considered insufficient to draw conclusions. In short: timing can help, but it is no guarantee, and the evidence is not conclusive.

Regular sexual intercourse throughout the week is an alternative to tests. It can make it easier to cover the period without a detailed schedule. Which approach works in everyday life varies.

When might it be time to seek medical care?

It often takes time to become pregnant, even when everything is working as it should. 1177 recommends that you contact a gynecological clinic or youth clinic if you have been trying to become pregnant for six months to a year without success, if you have not had a period in the last six months without being pregnant or in menopause, or if you have had three consecutive miscarriages. Seeking help is not about something being wrong with you—it is about getting an investigation that can provide answers.

Frequently asked questions

Can I become pregnant immediately after ovulation?
The chance drops quickly once ovulation has passed. But a calculated date can be wrong, so the calendar cannot give a certain answer that the fertile period is over.

Does an irregular period mean I do not ovulate?
Not necessarily. Some people often have irregular periods without anything being wrong. But long intervals between periods or completely missing periods can be signs that ovulation is not occurring, and it may be worth discussing with a healthcare provider.

Are ovulation tests better than counting days?
The Cochrane review from 2023 suggests that urine-based ovulation tests likely increase the chance of pregnancy compared to no prediction at all, but the results come partly from manufacturer-funded studies. Many manage just as well with regular sexual intercourse a couple of times a week.

This article provides general information and is not medical advice. It does not replace an individual assessment. Consult a midwife, doctor, or other healthcare professional if you have questions about your own health, your cycle, or your chances of becoming pregnant.

Sources

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