Reverse Due Date • Gestational Age Calculator

Conception Calculator:
When Did I Get Pregnant?

Enter your doctor's estimated due date or ultrasound date to work backward and identify your exact conception date, intercourse window, and estimated last menstrual period.

Enter Your Pregnancy Details

Estimated Conception Date
Calculating...
Probable Intercourse Window: Calculating...
Conception Milestone
38 Weeks
Due Date minus 266 days (fetal age).
Estimated Last Period
--
Due Date minus 280 days (clinical LMP).
Current Gestation
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Weeks & days pregnant today.

How Does a Conception Calculator Work?

A standard full-term human pregnancy lasts approximately 280 days (40 weeks) counted from the first day of the last menstrual period (LMP), or 266 days (38 weeks) counted from fertilization and ovulation.

Because fertilization can only occur after an egg is released (which lives approximately 12–24 hours), your conception date is virtually identical to your ovulation date. However, because healthy sperm can survive inside the female reproductive tract for up to 5 days, the sexual intercourse that caused conception could have occurred anywhere from 5 days prior to ovulation up to the day of ovulation itself.

Conception Date vs Ultrasound Dating

If you have had a first-trimester dating ultrasound (specifically crown-rump length measurement between 8 and 13 weeks), your ultrasound due date is considered more clinically accurate than any calendar estimation. If your doctor revised your due date based on an early scan, use that revised date in the calculator above to find the most accurate conception window.

Medically Reviewed by Dr. Sarah Jenkins, MD, FACOG ✓ Board-Certified OB-GYN

Clinical Review: October 29, 2026 • Adheres to ACOG Committee Opinion No. 700 (Methods for Estimating the Due Date)

ACOG guidelines state that gestational age dating based on ultrasound measurement in the first trimester supersedes menstrual dating when the discrepancy exceeds 5 to 7 days.

Medical Notice: Only about 4% of babies are born on their exact estimated due date. Most full-term deliveries take place between 37 and 41 completed weeks. This calculator provides statistical estimations for informational purposes.