How to Calculate Conception Date
What is Conception Date?
The conception date calculator works backwards from the estimated due date to estimate when conception occurred. It subtracts 266 days (38 weeks) from the due date, as fertilisation to birth averages 266 days. The last menstrual period is estimated by subtracting 280 days (40 weeks) from the due date.
Formula
Step-by-Step Guide
- 1Estimated conception = Due date − 266 days
- 2Estimated LMP = Due date − 280 days
- 3Gestational age from LMP = 40 weeks; from conception = 38 weeks
- 4Conception is typically 2 weeks after LMP in a 28-day cycle
Worked Examples
Frequently Asked Questions
What is Conception Calc?
The conception date calculator works backwards from the estimated due date to estimate when conception occurred. It subtracts 266 days (38 weeks) from the due date, as fertilisation to birth averages 266 days
How accurate is the Conception Calc calculator?
The calculator uses the standard published formula for conception calc. Results are accurate to the precision of the inputs you provide. For financial, medical, or legal decisions, always verify with a qualified professional.
What units does the Conception Calc calculator use?
This calculator works with inches. You can enter values in the units shown — the calculator handles all conversions internally.
What formula does the Conception Calc calculator use?
The core formula is: Estimated conception = Due date − 266 days. Each step in the calculation is shown so you can verify the result manually.
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