How Ovulation Works
Follicular phase (Day 1 to ovulation). Day 1 is the first day of full menstrual flow. Falling progesterone and estrogen trigger the pituitary to release follicle-stimulating hormone (FSH), which recruits a cohort of antral follicles. One dominant follicle emerges and secretes rising estradiol. Estradiol thickens the endometrium and thins cervical mucus. This phase is variable and accounts for nearly all cycle-length differences between women.
Ovulation. When estradiol stays high enough for long enough, it triggers a luteinizing hormone (LH) surge. Ovulation follows roughly 24–48 hours after surge onset (about 10–12 hours after the LH peak).
Luteal phase (ovulation to next period). The ruptured follicle becomes the corpus luteum and secretes progesterone, which stabilizes the endometrium. If no pregnancy occurs, the corpus luteum regresses and menstruation begins. This phase is relatively constant at 12–14 days in most women.
The Ovulation Formula
Ovulation day = Cycle length − Luteal phase length (default 14 days)
| Cycle Length | Estimated Ovulation Day | Fertile Window (Cycle Days) |
|---|---|---|
| 24 days | Day 10 | Days 5–10 |
| 28 days | Day 14 | Days 9–14 |
| 32 days | Day 18 | Days 13–18 |
| 35 days | Day 21 | Days 16–21 |
The fertile window is the five days before ovulation plus ovulation day. Conception probability is highest in the two days before ovulation and on ovulation day itself.
Worked example: LMP is March 10, and the average cycle is 32 days. Ovulation day = 32 − 14 = cycle day 18, which is March 27. The fertile window runs March 22–27.
Physiological Signs of Ovulation
- Cervical mucus: Under estradiol, mucus becomes clear, slippery, and stretchy, like raw egg white. The last day of this mucus (“peak day”) usually falls within a day or two of ovulation. Mucus turns thick and scant after ovulation under progesterone.
- Basal body temperature (BBT): Progesterone raises resting temperature by about 0.3–0.6 °C (0.5–1.0 °F) after ovulation, sustained for at least three days. BBT confirms ovulation retrospectively and cannot predict the fertile window in the current cycle.
- LH surge: Urine LH strips detect the surge 24–48 hours before ovulation. A positive result means ovulation is imminent, not complete.
- Other symptoms: Unilateral lower abdominal pain (mittelschmerz), mild mid-cycle spotting, increased libido, and breast tenderness occur in some women. They are inconsistent and not reliable on their own.
Ovulation Tracking Methods Compared
| Method | When to Test / Check | Accuracy Rate | Best For |
|---|---|---|---|
| Calendar / App Calculation | Predicted from LMP and cycle length before the cycle begins. |
Low to Moderate
Gives an estimated window, not a confirmed event, and is unreliable when cycle length varies by more than a few days.
|
Regular cycles; initial planning. |
| LH Urine Test Strips | Once or twice daily starting ~3–5 days before the predicted ovulation day; test until positive. |
High (~97–99%)
Detects the LH surge in validation studies; ovulation follows in roughly 24–48 hours. Can be falsely elevated in PCOS.
|
Pinpointing the best days to conceive. |
| Cervical Mucus Monitoring | Daily observation from the end of menses. |
Moderate
Identifies the fertile window well in trained users but is subjective and affected by infection, lubricants, and medications.
|
No-cost daily monitoring; combining with other methods. |
| Basal Body Temperature (BBT) | Every morning before rising, at the same time, after at least 3–4 hours of sleep. |
Moderate (Retrospective)
Retrospective confirmation only; disrupted by illness, alcohol intake, and shift work patterns.
|
Confirming ovulation occurred; learning your luteal phase length. |
Accuracy varies by study design and population. Combining LH testing with mucus observation gives the best practical result.
Irregular Cycles, PCOS, and Calculator Limitations
A calendar formula assumes a predictable luteal phase and a stable follicular phase. Neither holds with irregular cycles. In polycystic ovary syndrome (PCOS), oligo-ovulation or anovulation means the follicular phase may last weeks or never reach ovulation. Averaging cycle lengths then produces an estimated date with little predictive value.
- LH strips can give false positives in PCOS because baseline LH is chronically elevated.
- BBT shows no biphasic pattern in anovulatory cycles, which can itself signal anovulation.
- Follicle-tracking ultrasound and mid-luteal progesterone testing are clinical confirmation tools.
Women with cycles shorter than 21 or longer than 35 days should evaluate ovulation with a clinician. ASRM and ACOG recommend evaluation after 12 months of trying for women under 35, after 6 months for women 35–39, and promptly for women 40 and older or with known irregular ovulation.
For due-date calculation with irregular cycles, see Irregular Periods Due Date Calculator.
Frequently Asked Questions
Can you get pregnant outside your fertile window?
Pregnancy is very unlikely outside the six-day window but not impossible. Ovulation timing varies within and between cycles, and in cohort data only about 30% of women had a fertile window entirely within cycle days 10–17 (Wilcox et al., 2000). Early or delayed ovulation, shortened cycles, and the five-day sperm lifespan all shift the window from calendar estimates. Intercourse after ovulation carries a negligible conception probability.
How many days after my period do I ovulate?
It depends on period length and cycle length. In a 28-day cycle with 5 days, ovulation falls about 9 days after bleeding stops. For a 35-day cycle with 5 days, ovulation is about 16 days after the period ends. Count from Day 1 and use cycle length minus 14.
How accurate is a calendar-based ovulation calculation?
It is an estimate. Luteal phase length varies from about 10 to 16 days between women, and ovulation shifts by several days in many individual cycles. For regular cycles, a calendar calculation usually brackets the fertile window within a few days. For irregular cycles, accuracy is poor. LH testing improves precision because it detects a physiological event.
What happens to your due date if you conceived earlier or later than day 14?
Naegele’s rule assumes day-14 ovulation. If ovulation occurred on day 21, the pregnancy is 7 days “younger” than the LMP-based dating suggests. A first-trimester ultrasound will measure about 7 days smaller, and since that exceeds the 5-day threshold before 9 weeks, the due date is typically moved 7 days later. See conception date vs. LMP.