In more detail
Ovulation is triggered by a surge of luteinising hormone (LH), usually 24 to 36 hours beforehand, which is what home ovulation tests detect. The phase after ovulation is fairly constant at about 12 to 14 days, while the phase before it varies, so ovulation is better estimated as about 14 days before the next period than as day 14 of the cycle. The egg survives roughly 12 to 24 hours and sperm up to about five days, which gives a fertile window from five days before ovulation to the day after. A small rise in basal body temperature and changes in cervical mucus are other signs.
Why it matters
Knowing the window helps when trying to conceive. Calendar estimates are not reliable contraception: cycles shift with stress, illness, breastfeeding and age, and some people ovulate irregularly. Irregular cycles, or difficulty conceiving, are worth discussing with a doctor.
Example
28-day cycle, period starts 1 October 2026
Next period ≈ 29 October → ovulation ≈ 29 Oct − 14 days = 15 October
Fertile window: 10 to 16 October
30-day cycle from 1 September 2026: ovulation ≈ 17 September, window 12 to 18 September
Common confusion
Pregnancy is dated by Gestational age, counted from the first day of the last period, so it is about two weeks ahead of the time since ovulation and conception.