Ovulation is the release of a mature egg (oocyte) from the ovarian follicle, triggered by a surge in luteinizing hormone (LH). In a standard 28-day cycle, ovulation occurs around day 14 (counting from the first day of the last menstrual period). In cycles of varying length, ovulation typically occurs 14 days before the next period — so a 35-day cycle produces ovulation around day 21. Only one egg is typically released per cycle.

The fertile window spans approximately 6 days: the 5 days before ovulation plus ovulation day itself. This window exists because sperm can survive in the female reproductive tract for up to 5 days, while the egg remains viable for only 12–24 hours after release. The two most fertile days are ovulation day and the day before. Timing intercourse within this window maximizes conception probability.

Calendar methods (like the one used here) predict ovulation by estimating it occurs 14 days before the next expected period. Accuracy is high for women with regular 26–32 day cycles but drops significantly for those with irregular cycles. Studies show calendar-based methods have a 5–15% perfect-use failure rate as contraception. For conception planning, confirm prediction with ovulation predictor kits (OPKs) detecting the LH surge, or basal body temperature (BBT) tracking.

The LH (luteinizing hormone) surge is a sharp rise in LH secreted by the pituitary gland that triggers ovulation 24–36 hours later. Ovulation predictor kits (OPKs) detect urinary LH levels; a positive OPK indicates ovulation is imminent. The surge typically lasts 12–48 hours. Testing urine around 10 am–2 pm daily starting day 10–12 of the cycle gives the most reliable results for most women.

Basal body temperature (BBT) rises 0.2–0.5°C (0.4–1.0°F) after ovulation due to the thermogenic effect of progesterone. Charting BBT daily (before getting out of bed, using a basal thermometer) reveals a biphasic pattern: lower pre-ovulation, higher post-ovulation. The BBT method confirms ovulation has occurred but cannot predict it in advance, which is why combining BBT with OPKs or cervical mucus observation is more effective for timing conception.

Cervical mucus changes throughout the cycle under the influence of estrogen and progesterone. At the start of the cycle: dry or minimal discharge. Approaching ovulation: mucus becomes wetter, clearer, and more elastic — resembling raw egg whites (called fertile-quality or EWCM). This stretchy, clear mucus supports sperm survival and motility. After ovulation: mucus thickens and becomes opaque under progesterone influence. The 'egg-white' days are the most fertile.

Standard ovulation calculators assume a fixed cycle length and are less reliable for irregular cycles. Women with cycles varying ±7 days or more may experience significant prediction errors. Polycystic ovary syndrome (PCOS), thyroid disorders, and high stress can cause cycle irregularity and anovulatory cycles (no ovulation). For irregular cycles, OPKs (used from the earliest possible ovulation day) and BBT charting are more useful than calendar methods.

The luteal phase — from ovulation to the next period — typically lasts 12–16 days and is relatively fixed within an individual (usually 14 days). It is the pre-ovulatory follicular phase that varies in length, causing cycle-to-cycle differences. A luteal phase shorter than 10 days (luteal phase deficiency) may impair implantation due to insufficient progesterone. If you suspect a short luteal phase, discuss with a reproductive endocrinologist.

Ovulation can be disrupted by: PCOS (polycystic ovary syndrome), hypothyroidism or hyperthyroidism, hyperprolactinemia, extreme caloric restriction or excessive exercise (hypothalamic amenorrhea), BMI <18.5 or >35, chronic psychological stress, perimenopause, and certain medications (hormonal contraceptives, antipsychotics, chemotherapy). Approximately 25–30% of female infertility is ovulatory dysfunction. Evaluation by a gynecologist is recommended if cycles exceed 35 days regularly.

No. Even in women with regular cycles, ovulation timing can shift by ±2–5 days from cycle to cycle. Stress, illness, travel, and disrupted sleep (affecting circadian rhythms) can delay ovulation by days to weeks. Research using urinary hormone monitoring shows that day 14 is ovulation day in fewer than 10% of cycles; for most women ovulation ranges from day 11 to day 21. This is why fixed-date assumptions alone are insufficient for precise fertility tracking.

Ovulation is the release of the egg from the ovary (~day 14). If fertilized by sperm, the resulting embryo travels down the fallopian tube over 3–5 days and reaches the uterus. Implantation — when the embryo embeds in the uterine lining — occurs 6–12 days after fertilization (approximately days 20–26 of a 28-day cycle). Some women experience 'implantation cramping' or light spotting during this window, which can be confused with an early period.

An ovulation calendar estimates your fertile window and predicted ovulation day by counting forward from the first day of your last period, using your typical cycle length (the calendar method used by this calculator). This tool's Calendar tab lays that estimate out visually, day by day, alongside the cycle's phases. Like all calendar-based prediction, it assumes a fairly consistent cycle length and works best for cycles in the 26–32 day range — see the accuracy answer above for how reliability changes with irregular cycles.

Conception probability rises through the fertile window and peaks around ovulation day: for healthy couples, intercourse on the day of ovulation is associated with roughly a 33% chance of conception in that cycle, with probability increasing as ovulation approaches and dropping off quickly afterward, since the egg is viable for only 12–24 hours. These are population-level averages, not a guarantee for any individual cycle — actual odds vary with age, fertility, and timing accuracy.