How the Predictions Work
The calculator adds your cycle length to the first day of your last period to predict the next period start date. Ovulation is estimated at 14 days before the next period (luteal phase is relatively constant at 14 days for most women, even when cycle length varies). The fertile window spans 5 days before ovulation through ovulation day.
Menstrual Cycle Phases
| Phase | Days (28-day cycle) | What Happens |
|---|---|---|
| Menstruation | Day 1-5 | Uterine lining sheds. Bleeding occurs. |
| Follicular | Day 1-13 | Follicles develop in ovaries. Estrogen rises. |
| Ovulation | Day 14 | Mature egg is released. Fertility peaks. |
| Luteal | Day 15-28 | Progesterone rises. Lining thickens. PMS symptoms may start. |
When Cycles Are Irregular
Irregular periods (varying by more than 7 days cycle to cycle) make calendar-based predictions unreliable. Common causes include stress, significant weight changes, polycystic ovary syndrome (PCOS), thyroid disorders, and perimenopause. If your cycle is consistently outside 21-35 days or varies widely, consider tracking with basal body temperature or ovulation test strips for more accuracy.
Tracking for Conception
The fertile window is shortβabout 6 days per cycle. Sperm survive up to 5 days, but an egg only survives 12-24 hours after ovulation. The highest conception probability is the day before and the day of ovulation. Ovulation predictor kits (OPKs) detect the LH surge that precedes ovulation by 24-36 hours and are more accurate than calendar methods alone.
Disclaimer: This calculator provides estimates only. It is not a form of birth control and should not be relied upon for pregnancy prevention. Consult a healthcare provider for medical advice.