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Conception Date Calculator Guide
This works out roughly when conception happened, either from the first day of your last period or backwards from a due date.
From your period: Conception ≈ (Day 1 of last period + cycle length) − 14
From a due date: Conception ≈ Due date − 266 days
Both land on the same day for a 28-day cycle, because a due date is set 280 days from your last period and pregnancy itself lasts about 266 days from conception.
The 14 Days You Were Never Pregnant For
Here's the oddity at the centre of all pregnancy dating. Your pregnancy is counted from the first day of your last period — about two weeks before conception could possibly have happened.
So at "4 weeks pregnant" the embryo is around 2 weeks old. It's not a mistake; it's a convention. Nobody knows the day they ovulated, but almost everyone knows when their last period started, so medicine dates from the thing you can actually observe.
Why It's a Window, Not a Date
Conception can't be pinned to a day from a calendar, and honest tools don't pretend otherwise.
Sperm survive up to five days. So intercourse on the 10th can produce conception on the 15th. The date this calculator gives is the most likely single day, but the window spanning the five days before ovulation and the day after is where the truth actually lives.
If Your Cycle Isn't 28 Days
The standard formula assumes 28 days and quietly misleads everyone else. On a 35-day cycle you ovulate about a week later than the textbook, so conception was later — and a due date calculated the standard way is about a week early. Enter your real cycle length and the maths adjusts.
An Early Scan Beats All of This
A dating ultrasound in the first trimester measures the embryo directly and is considerably more accurate than any calculation from dates — especially with irregular cycles. If your scan date and your calculated date disagree, the scan usually wins. Our ultrasound due date calculator works from a CRL measurement.
A Note on Using This to Settle Questions
People sometimes reach for a conception calculator to work out paternity. It cannot answer that. The window is at least six days wide, ultrasound dating has its own margin of error of several days in early pregnancy, and neither is evidence of anything. Only a DNA test answers that question.
See also the due date calculator and pregnancy week calculator.
This is an estimate for general information, not medical advice. Every pregnancy and every cycle is different, and only your doctor or midwife can give you dates and guidance for your situation. Nothing here replaces professional care.
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