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Conception Calculator

Estimate your conception date and fertile window from your due date or last menstrual period.

Conception Estimate

About the Conception Calculator

The Conception Calculator helps you estimate when conception likely occurred, either by working backwards from a known due date or forwards from your last menstrual period. This can help you understand your fertile window and ovulation timing.

How to Use

  1. Choose From Due Date (if you know your due date) or From LMP (if you know your last period).
  2. Enter the relevant date.
  3. Adjust the cycle length if your cycle differs from 28 days.
  4. Click Calculate to see the estimated conception window.

How It Works

From Due Date: LMP = Due Date − 280 days − (cycleLen − 28). Ovulation ≈ LMP + (cycleLen − 14). Conception window = ovulation day ±3 days (sperm can survive ~5 days, egg ~24 hours).

From LMP: Ovulation ≈ LMP + (cycleLen − 14). Conception window = ovulation day ±5 days.

Typically, conception occurs 11–21 days after the first day of LMP (for a 28-day cycle, most likely around day 14).

Example

Due Date: December 1, 2024 → LMP ≈ February 25, 2024 → Ovulation ≈ March 10, 2024 → Conception window: March 7–15, 2024.

Frequently Asked Questions

Conception date estimates have a window of ±5–7 days due to variation in ovulation timing, sperm viability (3–5 days), and egg viability (~24 hours). The estimate is a range, not an exact date.
The fertile window is approximately the 6 days ending on the day of ovulation. This includes the 5 days before ovulation (while sperm can survive) and ovulation day itself. Conception is most likely in the 2–3 days before and on ovulation day.
The luteal phase (post-ovulation) is relatively fixed at ~14 days. Variation in cycle length is mostly in the follicular phase before ovulation. A 32-day cycle means ovulation around day 18, not day 14.