Ovulation Calculator

The menstrual cycle consists of two phases separated by ovulation: **Follicular phase** (variable length): From the first day of menstruation to ovulation. The hypothalamus releases GnRH, stimulating FSH secretion from the pituitary. FSH promotes follicle development in the ovary. A dominant follicle emerges and produces increasing estrogen, which thickens the endometrium. This phase varies in length — from 10-21+ days — accounting for cycle length variation. **Ovulation**: A surge of luteinizing hormone (LH), triggered by rising estrogen, causes the dominant follicle to rupture and release its egg. Ovulation occurs approximately 24-36 hours after the LH surge. This is the basis for ovulation predictor kits (OPKs) that detect the LH surge in urine. **Luteal phase** (relatively constant): From ovulation to menstruation. The ruptured follicle becomes the corpus luteum, producing progesterone to maintain the endometrium. This phase is remarkably consistent at 12-16 days (average 14) across women and cycles. This constancy is why we estimate ovulation as cycle length minus 14. **Conception probability by day**: - 5 days before ovulation: ~4% - 4 days before: ~8% - 3 days before: ~15% - 2 days before: ~20% - 1 day before: ~25-30% - Day of ovulation: ~25-30% - Day after ovulation: ~0-5% (egg viability declining) **Limitations**: This calculator assumes regular cycles. Women with irregular cycles, PCOS, or anovulatory cycles may not ovulate predictably. Basal body temperature charting, cervical mucus monitoring, and OPKs provide more real-time ovulation detection.