Conception Calculator
Estimate the date of conception by using your last menstrual period (LMP) or your due date. A useful reverse due date calculator for pregnancy planning.
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Pinpointing Your Conception Date
A conception calculator, often called a "reverse due date calculator," helps estimate the date when conception most likely occurred. This can be insightful for expectant parents who are curious about the timeline of their pregnancy. The calculator works backward from an estimated due date or forward from the first day of the last menstrual period (LMP).
How the Calculation Works
The calculation is based on a few key assumptions about the menstrual and pregnancy cycle:
- Standard Pregnancy Duration: A full-term pregnancy is estimated to last 40 weeks (280 days) from the first day of the last menstrual period (LMP).
- Ovulation Timing: Conception is only possible during a fertile window, which is centered around ovulation. In an average 28-day cycle, ovulation typically occurs around day 14.
Method 1: Using Your Last Menstrual Period (LMP)
This is the most common method used by healthcare providers. If you provide the first day of your LMP and your average cycle length, the calculator estimates your ovulation and conception date.
Estimated Conception Date ≈ LMP Date + (Cycle Length / 2)
Or more simply: LMP Date + ~14 days
This provides a probable range for conception, as sperm can survive in the female reproductive tract for up to 5 days, and the egg is viable for about 12-24 hours after ovulation.
Method 2: Using Your Due Date
If you have already been given an estimated due date (EDD) by your doctor, the calculator can work backward to find the likely conception date.
Estimated Conception Date ≈ Due Date - 266 days
Why 266 days? Because the 40-week (280-day) pregnancy term is counted from the LMP, not the date of conception. The time from conception to birth is actually about 38 weeks (266 days).
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