How ovulation is estimated
The second half of the cycle, from ovulation to the next period (the luteal phase), is fairly steady at about 14 days for most people. So ovulation is counted back from the expected next period:
Example: with a 28-day cycle and a period starting March 1, ovulation is likely around March 15 (day 15). The fertile window runs from about March 10 to March 16, and the next period is due March 29. With a 32-day cycle, ovulation moves to about day 19.
The fertile window is six days long because sperm can survive up to five days, and an egg lives about a day after release. The two or three days before ovulation are the most likely to lead to pregnancy.
Making the estimate better
- Track several cycles and use the average length. If yours varies by more than a week, a calendar estimate is much less reliable.
- Ovulation tests (LH strips), cervical mucus changes and a rise in basal body temperature help confirm the timing.
- If you have very irregular cycles or have been trying to conceive for a year (six months if over 35), talk to a doctor.
Frequently asked questions
When do I ovulate with a 28-day cycle?
Around day 14 or 15, counting the first day of your period as day 1. The fertile window is roughly days 9 to 15.
Can I use this as birth control?
No. Cycle timing varies and calendar estimates aren't reliable enough to prevent pregnancy.
How long does the fertile window last?
About six days: the five days before ovulation and the day of ovulation, with the highest chances in the last three.
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