How the predictions are made
- Next period: the first day of your last period plus your usual cycle length.
- Ovulation: the second half of the cycle (the luteal phase) is fairly steady at about 14 days, so ovulation is estimated as 14 days before the next period. In a 28-day cycle that is day 14; in a 32-day cycle, day 18.
- Fertile window: sperm can live for up to 5 days and an egg for about a day, so the fertile days are the 5 days before ovulation, ovulation day and the day after.
- Due date: 280 days (40 weeks) from the first day of your last period, adjusted for cycles longer or shorter than 28 days (Naegele's rule). An early ultrasound scan gives a more accurate date.
What counts as a normal cycle?
| Feature | Usual range for adults | See a health worker if |
|---|---|---|
| Cycle length | 21 to 35 days (up to 45 days in the first years after periods start) | Cycles are often shorter than 21 or longer than 35 days, or vary by more than 7 to 9 days |
| Bleeding | 2 to 7 days | Bleeding lasts more than 7 days |
| Amount | Changing pads or tampons every few hours | You soak through a pad or tampon every 1 to 2 hours, pass large clots, or feel dizzy or breathless |
| Missed periods | Can happen with stress, illness or weight change | No period for 3 months (and not pregnant), or none by age 15 |
| Bleeding at other times | Light spotting around ovulation can be normal | Bleeding between periods, after sex, or after the menopause |
Get help urgently if
- You could be pregnant and have pain on one side of your lower tummy, shoulder-tip pain or bleeding: this can be an ectopic pregnancy.
- Bleeding is very heavy and you feel faint, or your heart is racing.
For students & clinicians (technical detail)
- EDD by Naegele's rule = LMP + 280 days + (cycle length − 28) days. Dating by crown–rump length in the first trimester supersedes LMP dating when they differ by more than 5 to 7 days.
- Gestational age shown = days since LMP, expressed as weeks + days.
- Calendar-based fertility awareness methods have typical-use failure rates far above modern contraception.
- FIGO normal limits: frequency 24 to 38 days, duration ≤ 8 days, cycle-to-cycle variation ≤ 7 to 9 days.