Conception Calculator

Estimate conception date and fertile window from your last menstrual period and cycle length, or work backwards from a known due date.

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Enter the first day of your last period to estimate conception date.

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Health — Fertility

Conception Calculator

Estimating when conception likely occurred can help with early pregnancy dating, understanding a due date, or thinking back on a known pregnancy outcome. This calculator estimates conception date from either the first day of your last menstrual period or from a known due date.

How conception date is estimated from LMP

Ovulation typically occurs around 14 days before the next expected period — meaning in a standard 28-day cycle, ovulation falls on day 14 from the last menstrual period (LMP). Conception requires fertilisation, which happens within 12–24 hours of ovulation. Sperm can survive in the fallopian tubes for up to 5 days, so the fertile window extends from roughly 5 days before ovulation to 1 day after. The estimated conception date is the likely ovulation day; actual conception could be a few days earlier.

How conception date is estimated from due date

A full-term pregnancy is typically 40 weeks from the LMP, or approximately 38 weeks (266 days) from conception. Working backwards from a due date subtracts 266 days to estimate the approximate conception date. Because due dates themselves carry a degree of uncertainty, the estimated conception window is displayed as a range of approximately ±5 days.

Cycle length matters

Longer cycles shift the fertile window later. A woman with a 35-day cycle ovulates around day 21 (35 − 14), putting her estimated conception date significantly later than for a 28-day cycle. Shorter cycles shift ovulation earlier. If your cycle is irregular, average cycle length provides a useful approximation, but individual variation means exact dating requires ultrasound confirmation.

Frequently asked questions

Can the conception calculator tell me exactly when I conceived?

No. These are estimates based on population averages for cycle length and ovulation timing. Individual cycles vary, ovulation can shift, and early pregnancies are most accurately dated by an early ultrasound (before 14 weeks).

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