Pregnancy Conception Calculator
Estimate the likely date (and window) of conception from your due date, LMP, ultrasound findings or IVF transfer details.
Most likely conception date
Tuesday, December 2, 2025
Probable window opens
Thursday, November 27, 2025
Probable window closes
Wednesday, December 3, 2025
Conception dates are estimates only - sperm survive up to five days, so the true conception date can fall anywhere within the fertile window.
How to Use the Pregnancy Conception Calculator
- Choose your reference: due date, first day of last period, ultrasound date, or IVF transfer.
- Enter the relevant date(s).
- Read the estimated conception date plus a probable 5-day window around it.
Formulas Used
From due date: conception ≈ due date − 266 days
From LMP: conception ≈ LMP + 14 days (typical ovulation)
Ultrasound: conception ≈ scan date − (gestational age in days − 14)
Frequently Asked Questions
Can a calculator tell the exact day I conceived?
No. Sperm can survive up to five days and ovulation timing varies, so results are estimates. Even an exact known intercourse date may not be the conception date.
How does IVF change the calculation?
IVF removes uncertainty: for a day-5 blastocyst transfer conception is essentially the transfer date; for a day-3 embryo add two days to the transfer date.
Why subtract 266 days from the due date?
Gestation is counted as 280 days from the LMP, but conception occurs about two weeks into that count, leaving 266 days from conception to estimated delivery.
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