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Category Women\'s Health

First Day of Last Period (LMP)

Enter cycle lengths from the last 3-6 months (comma separated):

Example: If periods came 28 days apart, then 35 days, type: 28, 35

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About

Standard calculators fail women with irregular cycles by pinpointing a specific day that may be incorrect. If your cycle varies between 26 and 35 days, your ovulation day shifts drastically from month to month. This tool uses a statistical range approach. Instead of guessing a single date, it calculates a Widened Safety Window. It determines the earliest possible ovulation (based on your shortest cycle) and the latest possible ovulation (based on your longest cycle) to create a comprehensive fertile range. This ensures you cover all bases.

Formulas

We calculate the start and end of the "Active Trying" window.

{

Window_start = (Cycle_min − 14) − 5 daysWindow_end = (Cycle_max − 14) + 1 day

This results in a broader window (often 10-15 days) compared to the standard 6 days, compensating for irregularity.

Reference Data

Variable	Calculation Logic	User Impact
Earliest Ovulation	Shortest Cycle − 14 days	Defines start of fertile monitoring.
Latest Ovulation	Longest Cycle − 14 days	Defines end of fertile monitoring.
Safety Buffer	−5 days (Sperm Life)	Added to the earliest date.

Frequently Asked Questions

Enter at least 3 cycles for a baseline estimation. For better accuracy, entering 6 months of history helps identifying your true minimum and maximum cycle lengths.

Because we cannot predict exactly which cycle length you will have this month. By covering the entire range from your shortest to longest history, we ensure the actual ovulation day falls within the window.

Yes, this logic is superior for PCOS compared to standard apps. However, if you go months without a period, no calculator can predict ovulation. In that case, medical induction or OPK monitoring is necessary.