How We Calculate Conception
Pregnancy is conventionally counted as 40 weeks (280 days) from the first day of your last period (LMP). However, you don't actually get pregnant on that day. Conception typically happens about 2 weeks after your period starts, during ovulation.
Therefore, the "Reverse Due Date" method simply subtracts 266 days (38 weeks) from your estimated due date to find the most likely day the egg was fertilized.
The "Sperm Survival" Factor
Did you know?
You don't necessarily have to have sex on the *exact* day of conception to get pregnant. Sperm can survive inside the body for up to 5 days waiting for the egg.
This is why our calculator provides a Conception Window. If your calculated conception date is January 15th, intercourse leading up to that date (Jan 10-15) could have resulted in the pregnancy.
Accuracy: Ultrasound vs. LMP
Which date should you trust?
- Dating Scan (Ultrasound)
Most Accurate. An ultrasound between 8-12 weeks measures the embryo's crown-rump length (CRL), which is the gold standard for dating.
- Last Menstrual Period (LMP)
Less Accurate. It assumes you ovulated exactly on day 14 of a 28-day cycle. If you ovulated late, your calculation could be off by a week or more.
Frequently Asked Questions
Can this calculator prove paternity?
My baby was born early. Does that change my conception date?
I have irregular periods. Is this calculating correctly?
Tools & Data Verified by the EverydayCalculators Medical Research Team.
Last updated: January 2026.