About

Menstrual health extends beyond just ovulation. This tool adopts a holistic approach by tracking the entire cycle structure. Unlike generic calculators that assume a rigid 14-day luteal phase (the time between ovulation and the next period), this predictor allows users to input their specific Luteal Phase Length. This is critical for women tracking Basal Body Temperature (BBT) who know their phase differs from the average.

By customizing the luteal phase, the tool improves the accuracy of both the prediction for the next menstrual cycle start date and the identification of the ovulation window. It also screens for Luteal Phase Defect logic, alerting users if their post-ovulation phase is biologically too short (< 10 days) to sustain a pregnancy.

Formulas

The calculator determines the Next Period Date P_next simply by adding the Cycle Length C to the Last Period LMP:

P_next = LMP + C

However, the Ovulation Date O is calculated retroactively from the end of the cycle, using the user-defined Luteal Phase Length L_phase:

O = P_next − L_phase

Reference Data

Cycle Phase	Duration Range	Hormonal Dominance	Key Event
Menstruation	Days 1−5	Low Estrogen/Progesterone	Uterine lining shedding
Follicular Phase	Days 6−13	Rising Estrogen	Follicle maturation
Ovulation	Day 14 (Avg)	LH Surge	Egg release
Luteal Phase	Days 15−28	High Progesterone	Lining maintenance / PMS

Frequently Asked Questions

The luteal phase is the second half of your cycle, starting after ovulation and ending the day before your next period. While the first half (follicular phase) varies significantly between women, the luteal phase is usually consistent for a single individual, typically lasting 12-14 days.

If you entered a luteal phase of less than 10 days, the tool triggers a warning. A luteal phase shorter than 10 days (Luteal Phase Defect) may not provide enough time for a fertilized egg to implant properly in the uterine lining, potentially leading to fertility issues.

If unknown, leave the default value of 14. This is the medical average. To find your specific length, you can use ovulation test strips or track your temperature; the number of days from your positive test/temp shift until your period starts is your luteal phase length.

Yes. PMS symptoms typically manifest during the late luteal phase when progesterone levels drop. The tool highlights the 3-5 days preceding the predicted period start date as the likely window for PMS symptoms.