Understanding your cycle makes it easier to know what is normal for you, when you are most likely to conceive, and when something is worth getting checked.

What counts as a normal cycle

A cycle is counted from the first day of one period to the day before the next begins. Anything from 21 to 35 days is normal.

You will often see 28 days quoted. That is an average, not a standard — and it causes a lot of unnecessary worry. If your cycle is consistently 33 days, that is simply your cycle. What matters more than the number is whether it is reasonably predictable for you.

The four phases

  • Menstrual phase — the womb lining sheds; this is your period.
  • Follicular phase — FSH from the pituitary gland stimulates follicles in the ovary to grow, one of which becomes dominant. Oestrogen rises and rebuilds the womb lining.
  • Ovulation — a surge of luteinising hormone triggers the dominant follicle to release its egg.
  • Luteal phase — the empty follicle becomes the corpus luteum and produces progesterone, preparing the lining for a possible pregnancy. If there is no pregnancy, progesterone falls and the next period begins.

The follicular phase is what varies between women. The luteal phase is fairly constant at about 14 days.

When ovulation actually happens

This is the most useful thing to understand, and it is commonly got wrong.

Ovulation is roughly 14 days before your next period — not on day 14. Those are the same thing only if your cycle is exactly 28 days.

  • 28-day cycle → ovulation around day 14
  • 35-day cycle → ovulation around day 21
  • 24-day cycle → ovulation around day 10

Counting forward from your period will mislead you if your cycle is not 28 days. Counting back from the next one is more reliable.

Your fertile window

An egg survives about 24 hours after release. Sperm, however, can survive in the female reproductive tract for up to about 5 days.

This means the fertile window opens several days before ovulation and closes shortly after it — roughly a six-day span. Sex a few days before ovulation is at least as likely to result in pregnancy as sex on the day itself.

Our fertile period calculator will estimate this for your cycle.

Signs of ovulation

  • Cervical mucus becomes clearer, wetter and more slippery — often compared to raw egg white. This is the most useful everyday sign.
  • Basal body temperature rises slightly after ovulation. Useful for confirming it happened, but not for predicting it.
  • Mild one-sided pelvic pain around mid-cycle, in some women.
  • Ovulation predictor kits detect the LH surge in urine, giving a day or so of warning.

A word about not overdoing it

Couples often turn conception into a scheduling exercise, which is stressful and rarely necessary. Having sex every two to three days throughout the cycle is as effective as trying to hit a precise day, and considerably easier to live with.

Irregularities worth checking

  • Amenorrhoea — periods absent altogether
  • Cycles consistently shorter than 21 or longer than 35 days
  • Cycles that vary widely from month to month
  • Very heavy bleeding, or periods so painful they stop you doing normal things
  • Bleeding between periods or after sex

Irregular cycles often mean ovulation is not happening reliably — see our guide to anovulation. Common causes include PCOS, thyroid problems, weight change and stress.

When to see a doctor

See a doctor if you have been trying to conceive for a year, or six months if you are over 35 — and sooner if your periods are irregular or absent, or if you have any of the changes listed above.

The bottom line

A normal cycle is 21 to 35 days, and 28 is only an average. Ovulation happens about 14 days before your next period, not on day 14 — and your fertile window opens several days before that, because sperm survive. For most couples, regular sex across the cycle beats precision timing.

References

  1. National Health Service. Fertility in the menstrual cycle [Internet]. NHS; [cited 2026 Jul 22]. Available from: https://www.nhs.uk/conditions/periods/fertility-in-the-menstrual-cycle/
  2. National Health Service. Periods [Internet]. NHS; [cited 2026 Jul 22]. Available from: https://www.nhs.uk/conditions/periods/
  3. National Institute for Health and Care Excellence. Fertility problems: assessment and treatment. NG257. London: NICE; 2026. Available from: https://www.nice.org.uk/guidance/ng257