How to calculate your fertile window – when are you most likely to get pregnant?

Short answer: The chances of getting pregnant are highest during the roughly six days ending on the day of ovulation, and highest of all on the day before ovulation. Ovulation usually occurs about 14 days before the next period. Since cycles vary, the period is difficult to pinpoint exactly. Intercourse a couple of times a week covers the fertile window without you needing to calculate.

What is meant by the fertile window?

The fertile window is the days in the menstrual cycle when intercourse can lead to pregnancy. The reason the window extends several days back is that sperm can survive for a time in the uterus and fallopian tubes, while the egg is only viable for about 24 hours after ovulation. According to 1177 Vårdguiden, the chances are highest approximately five days before ovulation up to and including the day after, and sperm can survive for up to seven days.

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

How to estimate when you ovulate

A menstrual cycle is counted 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 the next period, which for many—but far from all—falls in the middle of the cycle.

The simplest method is to count 14 days back from the day you expect your period. The fertile window would then be approximately the five days before plus the day after. This method assumes you have reasonably regular cycles and can predict when your period will come—making it an estimate, not an exact date.

It is also worth knowing that the often-cited rule of thumb of days 10–17 of the cycle is less accurate than many believe. In the prospective study published in 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 significant probability of being fertile. The data thus suggest 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 discharge becomes clearer and stickier and may resemble raw egg white, mild pain in the lower abdomen, increased sex drive, and tender breasts. These signs are indicative but not reliable individually.

Ovulation tests from pharmacies measure the LH surge in urine and can indicate that ovulation is expected within approximately 24 hours. Basal body temperature measurement can show that ovulation has already occurred, as the temperature rises slightly afterward—making the method more useful for observing 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 intercourse actually help?

Here, the evidence is more nuanced than one might think. A 2023 Cochrane review, comprising seven randomized studies with a total of approximately 2,464 women or couples, found that timed 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 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 deemed insufficient to draw conclusions. In short: timing can help, but it is not a guarantee, and the evidence is not unequivocal.

Many couples achieve the same effect without tests by simply having intercourse about twice a week, which 1177 highlights as an alternative. In that case, there are usually live sperm present when ovulation occurs. For some, it can also be less stressful than counting days.

When might it be appropriate to seek care?

It often takes time to get 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 get pregnant for six months to a year without success, if you have not had a period for the past six months without being pregnant or in menopause, or if you have had three miscarriages in a row. Seeking help is not about something being wrong with you—it's about getting an investigation that can provide answers.

Frequently Asked Questions

Can I get pregnant immediately after ovulation?
The probability drops rapidly after ovulation because the egg is only viable for about 24 hours. 1177 includes the day after ovulation in the period when the chance is highest, but the evidence suggests that the chance thereafter is very small.

Does irregular menstruation mean I'm not ovulating?
Not necessarily. Some people often have irregular periods without anything being wrong. But long intervals between periods or completely missed periods can be signs that ovulation is not occurring, and it may be worth discussing with a healthcare professional.

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

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

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