Calculate your fertile days based on your last period date. Helps with pregnancy planning.
Guide to Ovulation
What is ovulation?
Ovulation is the process in which a mature egg is released from the ovary into the fallopian tube, where fertilization can occur. In a healthy woman with a regular cycle, ovulation typically occurs about 14 days before the start of the next period. During ovulation, luteinizing hormone (LH) levels rise, which can be detected using ovulation tests. Cervical mucus becomes more transparent and stretchy at this time, resembling "egg white" - a natural sign of fertility.
Fertile window
- 5 days before ovulation - sperm can survive in the female reproductive tract for up to 5 days
- Day of ovulation - highest chance of fertilization
- 1-2 days after ovulation - the egg remains viable for about 24-48 hours
How to increase pregnancy chances?
Regular intercourse every 2-3 days during the fertile window increases chances of conception. It's important to avoid harmful substances (alcohol, cigarettes), maintain a healthy body weight, and reduce stress. Folic acid supplementation is recommended before pregnancy. Tracking your cycle and observing fertility signs - basal body temperature, cervical mucus, and cervical changes - is beneficial. If pregnancy doesn't occur after a year of regular attempts, consult a doctor.
Irregular cycles
Menstrual cycles can be irregular due to many factors: stress, climate change, intense physical exercise, hormonal imbalances (PCOS, thyroid issues), or breastfeeding. With irregular cycles, predicting ovulation is more difficult, but it can still be monitored through LH tests, mucus observation, or basal temperature tracking. For very long or irregular cycles, consult a doctor.
The day-14 rule is right for one cycle length and wrong for every other
Ovulation is anchored to the next period, not the last one. The luteal phase after ovulation averages about 14 days regardless of cycle length, so a 35-day cycle ovulates near day 21 and a 24-day cycle near day 10. Counting to day 14 from your last period is correct only if your cycle happens to be 28 days — at 35 days it is off by a week.
How it works
- Estimates the likely ovulation day by counting back from the expected next period.
- Marks the fertile window, which ends at ovulation rather than centring on it.
- Widens the window when cycles vary, since variability is what makes prediction hard.
ovulation ≈ cycle length − 14 days fertile window ≈ the 6 days ending at ovulation sperm can survive up to about 5 days; the egg about 12–24 hours with a variable cycle: window runs from (shortest − 19) to (longest − 14)
Worked example
Where ovulation falls at different cycle lengths, then what happens when cycles vary.
- 24-day cycle → ovulation near day 10, window days 5–10
- 28-day cycle → ovulation near day 14, window days 9–14
- 32-day cycle → ovulation near day 18, window days 13–18
- 35-day cycle → ovulation near day 21, window days 16–21
- cycles ranging 26–32 days → combined window days 7–18
A six-day variation in cycle length turns a six-day fertile window into a twelve-day one. Irregular cycles do not shift the window — they widen it, which is why calendar prediction gets less useful exactly when people most want an answer.
Reading the result
- The 14-day luteal phase is an average, not a constant. It commonly runs from about 11 to 17 days between individuals, so even a perfectly regular cycle carries several days of uncertainty in where ovulation actually falls.
- The window ends at ovulation rather than surrounding it, which is the detail most calendars get wrong. Because sperm survive far longer than the egg, the days before ovulation matter more than the days after — by the day after, the opportunity has largely passed.
- Calendar prediction is not a contraceptive method. Even correctly applied, the uncertainty here is measured in days, and methods that rely on it have failure rates far above those of contraception designed for the purpose.
- Basal temperature confirms ovulation after it has happened; ovulation predictor kits signal it a day or so ahead. If timing matters to you, a direct signal beats arithmetic, because it responds to your actual cycle rather than to an assumed average.
Common questions
- Is ovulation always on day 14?
- Only on a 28-day cycle. Ovulation tracks the next period, not the last, so it falls near day 10 on a 24-day cycle and near day 21 on a 35-day one. Applying day 14 universally misses by up to a week.
- My cycles are irregular — can I still predict it?
- Less precisely, and the effect is a wider window rather than a shifted one. Cycles ranging 26 to 32 days produce a fertile span from day 7 to day 18. Where the answer matters, an ovulation test or temperature tracking gives a signal that calendar arithmetic cannot.