Ovulation & Pregnancy Probability Calculator

Calculate your fertile window and estimate pregnancy probability based on cycle length. Features a dynamic fertility calendar and statistical conception odds.

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Category Womens Health
Standard is 28 days
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About

Timing is the single most critical factor in conception. Conception is only possible during a small window of the menstrual cycle—specifically, the five days leading up to ovulation and the day of ovulation itself. This tool helps you identify that 'Fertile Window' with precision.

Using the standard Calendar Method (or Rhythm Method) adapted with statistical probability data from medical studies, this calculator estimates your date of ovulation based on your Last Menstrual Period (LMP). It goes beyond simple dates by providing the statistical probability of conception for each day of your window, helping you maximize your chances of success.

fertile days pregnancy probability conception calculator menstrual cycle

Formulas

The estimation relies on the luteal phase consistency. For a regular cycle:

Dateovulation ≈ DateNextPeriod − 14 days

The fertile window is defined as:

Window = [Dateovulation − 5, Dateovulation + 1]

Reference Data

Day Relative to OvulationFertility StatusConception Probability
Ovulation - 5 DaysFertile Window Starts~10%
Ovulation - 4 DaysHigh Fertility~16%
Ovulation - 3 DaysHigh Fertility~14%
Ovulation - 2 DaysPeak Fertility~27%
Ovulation - 1 DayPeak Fertility~31%
Ovulation Day (0)Ovulation Occurs~33%
Ovulation + 1 DayWindow Closes< 5%

Frequently Asked Questions

The calculation assumes a 'Luteal Phase' (time from ovulation to next period) of 14 days, which is the average for most women. However, this phase can range from 10 to 16 days. If you know your specific luteal phase length, the results will be more accurate.
It is highly unlikely. The egg survives for only 12-24 hours after release, and sperm can survive inside the body for up to 5 days. This 6-day overlap represents the only biological window for fertilization.
If your cycle length varies significantly (more than a few days) month to month, calendar calculators become less reliable. In such cases, tracking basal body temperature (BBT) or using ovulation predictor kits (OPKs) is recommended alongside this tool.