The fertile window: which days you can get pregnant

You can only get pregnant on a handful of days each cycle, and knowing roughly which ones helps if you are trying to conceive. The fertile window is easy to state but often misunderstood, especially the idea that ovulation happens on day 14. The ovulation calculator estimates the window from your cycle, and this guide explains where the numbers come from.

Why the window is about six days

Two lifespans set the window. An egg survives for roughly 12 to 24 hours after ovulation. Sperm, in fertile cervical mucus, can survive in the reproductive tract for up to about five days. Pregnancy is therefore possible from sex in the five days before ovulation and on the day of ovulation itself: about six days in total. The chance is highest in the two or three days leading up to and including ovulation, and falls sharply afterwards; a day after ovulation the chance is close to zero.

When ovulation happens

Ovulation usually occurs about 14 days before the next period, not 14 days after the last one. The phase after ovulation (the luteal phase) is fairly constant at about 12 to 16 days, while the phase before it varies. So in a 28-day cycle ovulation falls around day 14, in a 32-day cycle around day 18, and in a 25-day cycle around day 11. That is why calendar calculators ask for your cycle length: the estimate is the predicted start of the next period minus 14 days, with the fertile window the five days before that plus the day itself. If your cycles vary, the estimate varies with them; the period tracker averages your recent cycles to make it more realistic.

Signs and tests that pinpoint it

Your body gives clues that are more precise than a calendar:

• Cervical mucus becomes clear, slippery and stretchy, like raw egg white, in the most fertile days, then thickens after ovulation.
• Ovulation (LH) tests detect the hormone surge that triggers ovulation, typically 24 to 36 hours beforehand, so a positive test means the next day or two are the most fertile.
• Basal body temperature, taken on waking before getting up, rises by about 0.2 to 0.5 °C after ovulation and stays up until the period. It confirms ovulation has happened rather than predicting it, but over a few cycles it shows when you usually ovulate.

Some people also notice one-sided pelvic twinges around ovulation, though this is less reliable.

Timing for conception

For people trying to conceive, the usual advice is simpler than precise tracking: having sex every two to three days throughout the cycle covers the fertile window without the pressure of hitting exact days. If you prefer to focus on the window, sex every day or two in the days before ovulation works well, because sperm are already waiting when the egg is released. Most couples conceive within a year of regular unprotected sex; the NHS suggests seeing a GP if it has not happened after a year, or sooner if you are over 35 or know of a reason it might be harder. The due date calculator takes over once a test is positive.

Why it is not contraception

Calendar estimates are averages, and ovulation can shift with stress, illness, travel or simply from month to month. Fertility awareness methods that combine temperature, mucus and calendar tracking, taught properly and followed carefully, can be effective, but with typical use they fail far more often than most contraception: up to roughly one in four users become pregnant in a year. A calculator's fertile window should never be used on its own to avoid pregnancy. If avoiding pregnancy matters, talk to a doctor, nurse or sexual-health clinic about reliable options. How to track your period: what to log and what it tells you covers the dates worth recording either way.