Most fertile days: how to tell when you are ovulating

The six-day fertile window is the same for everyone in principle. What varies is where it falls: the day of ovulation moves from one cycle to the next and between people, and the most fertile days move with it.

The fertile window lasts six days and ends on the day of ovulation

In a study of 221 women who were trying to conceive, pregnancy followed intercourse only in the six days ending on the estimated day of ovulation (Wilcox, Weinberg and Baird, New England Journal of Medicine, 1995; 625 cycles). The chance of conception rose from about 10 in 100 when intercourse was five days before ovulation to about 33 in 100 when it was on the day of ovulation. The window is this long because sperm survive for up to five days in the reproductive tract, while an egg survives for about 12 to 24 hours after ovulation (American College of Obstetricians and Gynecologists, ACOG, patient article on trying to get pregnant, 2023). That is also why sex near the end of a period can occasionally lead to pregnancy, as the guide to can you get pregnant on your period explains.

UK and US sources give different figures for sperm survival. The NHS guidance on periods and fertility (2023) says sperm can survive for up to 7 days, which would extend the fertile time a little earlier in the cycle. The calculators on this site use ACOG's five days, which matches the six-day window measured in the 1995 study.

The two or three days before ovulation carry the highest chance of pregnancy

The studies agree that the chance is highest at the end of the window and differ slightly on the single best day. The American Society for Reproductive Medicine (ASRM), in its 2022 committee opinion on optimizing natural fertility, places peak fertility within the two days before ovulation, drawing on the same 221 women. A second study, which combined one group that dated ovulation by temperature with another that used urine hormones, found the chance greatest on the day before ovulation and falling on the day itself (as summarized by ASRM, 2022).

Counted by day of the cycle, ASRM (2022) reports that the chance of a clinical pregnancy rises from about 3 in 100 on cycle day 8 to about 9 in 100 on day 12, and falls below 2 in 100 by day 21. These figures are for women who described their cycles as generally regular.

The most fertile days move from cycle to cycle, even when cycles are regular

Calendar methods assume that ovulation falls 14 days before the next period, but the follicular phase, the days before ovulation, changes length from cycle to cycle. A later analysis of the same women tested that assumption (Wilcox, Dunson and Baird, BMJ, 2000; 696 cycles from 213 of the 221 women). Only about 30 in 100 women had their whole fertile window within days 10 to 17 of the cycle, the days older clinical guidance gave for a 28-day cycle. In cycles that lasted exactly 28 days, ovulation fell exactly 14 days before the next period in only about 1 in 10. On every day from day 6 to day 21, at least 10 in 100 women with regular cycles were in their fertile window, and women with regular 28-day cycles were most likely to be fertile on days 8 to 15.

Calendar apps rest on the same assumption. ASRM (2022) cites a study of 949 women in which calendar apps, compared with urine LH testing, identified the day of ovulation at best 21 times in 100. The ovulation calculator on this site shows its estimate with the full range that a normal luteal phase of 11 to 17 days allows (ASRM and SREI, luteal phase deficiency opinion, 2026), and it states that a calendar cannot detect ovulation in the current cycle.

Three signs show whether ovulation is near or has passed

These are the main ovulation symptoms; that guide covers the others and when each appears.

• Cervical mucus. Clear, slippery, stretchy mucus appears in the days before ovulation and peaks within the two to three days before it (ASRM, 2022). ACOG's patient guidance says that, of the methods for detecting ovulation yourself, monitoring cervical mucus tends to be the most reliable (ACOG, 2023). See cervical mucus through the cycle.
• A positive urine LH test. The surge in luteinizing hormone (LH) comes about one to one and a half days before ovulation (US Food and Drug Administration, 2018). See ovulation predictor kits.
• A rise in basal body temperature. A sustained rise of 0.5 to 1°F, about 0.3 to 0.6°C, follows ovulation (ACOG, 2019). It confirms ovulation after the event and cannot predict it. See basal body temperature.

Mucus and LH tests point forward to ovulation, and temperature points back to it. Used together over a few cycles, they show where the most fertile days tend to fall for one person rather than for an average cycle. With irregular cycles the picture is harder to read, as the page on ovulation and irregular periods explains.

US guidance times sex to the fertile window, and UK guidance spreads it across the cycle

ASRM (2022) finds the chance of pregnancy highest with intercourse every one to two days during the fertile window, and states that more frequent intercourse does not lower fertility. ACOG's patient guidance (2023) describes the same interval, every day or every other day in the six-day window. The NHS gives a figure for the whole cycle instead: its guidance on trying to get pregnant (2026) states that people aged 39 or under who have sex without contraception every 2 to 3 days are likely to conceive within a year. The authors of the 2000 BMJ study reached a similar conclusion, that intercourse two or three times a week avoids the uncertainty of predicting the fertile days. The guide to how to get pregnant fast sets the timing alongside the health steps.

The figures on this page come from couples without known fertility problems, and the authors of the 2000 study note that their estimates do not necessarily apply to couples having trouble conceiving.