When Am I Most Fertile?

What is the fertile window?

The fertile window is the stretch of each cycle when sex can lead to pregnancy. It exists because of a timing mismatch. Once an egg is released, it survives in the fallopian tube for only about 24 hours, according to ACOG. Sperm last much longer: usually about 3 days and sometimes up to 5 days, ACOG says, and the NHS gives up to 7 days in the fallopian tubes. So sperm that arrive a few days early can still be waiting when the egg appears.

The clearest measurement of this window comes from a study by Allen Wilcox and colleagues at the US National Institute of Environmental Health Sciences, published in the New England Journal of Medicine in 1995. They followed 221 women who were trying to conceive. The women collected daily urine samples so the researchers could pinpoint ovulation from hormone levels, and kept a daily record of when they had sex. Across 625 cycles, conception happened only when sex took place during a six-day period that ended on the estimated day of ovulation.

That six-day definition is still the one specialists use. The American Society for Reproductive Medicine (ASRM) says that, for counseling purposes, the fertile window is best defined as the six-day interval ending on the day of ovulation.

Which days give the best chance of pregnancy?

Within those six days, the odds are not equal. They rise as ovulation gets closer. Wilcox and colleagues reported a probability of conception of about 10% for sex five days before ovulation and about 33% for sex on ovulation day itself. The ASRM summarizes the same study this way: the highest chance came from sex within the two days before ovulation.

Day relative to ovulation	Chance of pregnancy from sex on that day	What the research says
6 or more days before	Very low	No conceptions outside the six-day window in the 1995 study
5 days before	About 10%	Start of the fertile window (Wilcox, 1995)
4 and 3 days before	Rising	Sperm can survive long enough to meet the egg
2 days and 1 day before	Among the highest	Peak odds were within the 2 days before ovulation (ASRM)
Ovulation day	About 33%	Last day of the six-day window (Wilcox, 1995)
1 day after	Low	The egg lives about 24 hours; ACOG counts this day as possible
2 or more days after	Very low	The egg is no longer available

These figures come from healthy volunteers trying to conceive. They describe chances per act of sex. They are not a forecast for any one person or any one cycle.

How do I find my fertile days in my own cycle?

Start with your cycle length: the number of days from the first day of one period to the day before the next. The NHS says regular cycles anywhere from 21 to 35 days long are normal. It also says ovulation usually happens about 10 to 16 days before the next period. So the fertile window moves with your cycle. It is not tied to day 14.

A common rough estimate puts ovulation about 14 days before the next period. ACOG explains that the number of days between ovulation and the next period is the most consistent part of the cycle, which is why estimates count back from your expected next period rather than forward from your last one.

Cycle length	Estimated ovulation	Estimated fertile window
24 days	Around day 10	About days 5 to 10
28 days	Around day 14	About days 9 to 14
32 days	Around day 18	About days 13 to 18
35 days	Around day 21	About days 16 to 21

Treat this table as a starting point. If your cycle length changes from month to month, use your shortest and longest recent cycles. Your window then covers a wider range of days. Our ovulation calculator does this arithmetic for you.

Why can’t I just count to day 14?

Because real cycles vary more than the textbook. In a 2000 follow-up analysis in the BMJ, Wilcox and colleagues looked at 696 cycles. Only about 30% of women had their fertile window entirely within days 10 to 17, the days clinical guidelines often point to. Most reached it earlier, and some much later. On every day between day 6 and day 21, women had at least a 10% chance of being in their fertile window. The authors concluded that the timing can be highly unpredictable, even for women whose cycles are usually regular.

Calendar predictions have the same limit. The ASRM cites a study of 949 volunteers in which calendar apps predicted the exact day of ovulation correctly at most 21% of the time, compared with urine hormone tests. A calendar estimate tells you roughly where to look. It cannot tell you the exact day.

How can I narrow down my fertile window?

Your body gives clues around ovulation. Combining them with a calendar estimate gives a much better picture:

• Cervical mucus. ACOG says that just before ovulation the cervix makes noticeably more mucus, and it becomes thin and slippery. The ASRM notes that the odds are highest when mucus is slippery and clear.
• Ovulation (LH) tests. These urine tests detect the hormone surge that triggers ovulation. The ASRM says ovulation may follow at any time within the next 2 days.
• Basal body temperature. Your resting temperature rises slightly, about 0.5 to 1°F, around ovulation and stays higher until your next period, ACOG explains. The most fertile days are the 2 to 3 days before the rise, so temperature confirms ovulation afterwards rather than predicting it.

Our guide to the signs of ovulation explains each of these in more detail.

How often should we have sex when trying to conceive?

The ASRM says the chance of pregnancy is highest with sex every 1 to 2 days during the fertile window. Couples should not be told to limit how often they have sex, though the ASRM also notes that strict schedules can add stress. Regular sex throughout the cycle also covers the fertile window when ovulation comes early or late. And patience is normal: the ASRM says about 80% of couples conceive within the first 6 months of trying.

Can I use my fertile window to avoid pregnancy?

Not on its own. The fertile window predictions on this page, in our calculator and in any period tracking app are estimates for understanding your cycle. They are not a method of birth control. Ovulation can shift from cycle to cycle, and the NHS notes that pregnancy soon after a period is possible with early ovulation or a short cycle. If you want to avoid pregnancy, talk to a doctor, nurse or midwife about contraception that suits you.

When should I talk to a doctor?

ACOG recommends an infertility evaluation if you have not become pregnant after 1 year of regular sex without birth control. If you are older than 35, ACOG recommends one after 6 months. If you are older than 40, it suggests talking with an ob-gyn about an evaluation straight away.

It is also worth talking to a doctor, nurse or midwife sooner if your periods are very irregular or stop for several months, or if anything about your cycle worries you. An appointment is a normal part of trying for a baby. It does not mean something is wrong.