What ovulation is
Each cycle, several follicles in your ovaries begin to grow, and usually one becomes dominant. A surge of luteinizing hormone (LH) then triggers that follicle to release its egg. The egg travels into the fallopian tube, where it can be fertilized for roughly 12 to 24 hours.
A cycle in two halves
The first half, from the first day of your period to ovulation, is the follicular phase. Its length varies from woman to woman and from cycle to cycle. The second half, from ovulation to your next period, is the luteal phase, and it is steadier at about 12 to 14 days.
That is why ovulation is easier to find by counting backwards. In a 28-day cycle it tends to fall around day 14. In a 32-day cycle it is closer to day 18. A typical cycle is anywhere from 21 to 35 days.
The fertile window
A landmark study in the New England Journal of Medicine followed couples who were trying to conceive and found that pregnancy happened only when they had sex during a six-day span ending on the day of ovulation. The chance was highest in the last two or three days of that span.
For couples who are trying, the American Society for Reproductive Medicine notes that sex every one to two days during the fertile window gives the best odds. There is no need to wait for the exact day.
How to spot it
Your body gives several signals. None is perfect on its own.
- Ovulation tests: urine strips that detect the LH surge, which arrives roughly a day or a day and a half before ovulation.
- Cervical mucus: it becomes clear, slippery and stretchy, a little like raw egg white, in the days before ovulation.
- Body temperature: your resting temperature rises slightly after ovulation. This confirms that it happened but comes too late to predict it.
- Apps: they estimate from past cycles. They work best when your cycles are regular.
When cycles are irregular
If your cycles are usually shorter than 21 days, longer than 35, or missing altogether, you may not be ovulating regularly. This is common and often treatable. Polycystic ovary syndrome and thyroid conditions are among the usual causes.
It is a good reason to see a doctor sooner rather than waiting a full year of trying.
When to get your fertility checked