How is the conception date estimated?

From a due date: subtract 266 days. From LMP: add about 14 days (adjusted for cycle length). From ultrasound: calculate LMP then add 14 days.

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Pregnancy Conception Calculator

Estimate when conception likely occurred based on your due date, last period, or ultrasound dating.

Privacy: All calculations run locally in your browser. No data is sent to any server.

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About This Calculator

This tool works backwards from pregnancy information you already have (due date, last period, or ultrasound) to estimate when conception most likely occurred.

How it's calculated

From a due date, the tool subtracts 266 days (38 weeks) to estimate ovulation/conception. From LMP, it adds about 14 days (adjusted for your cycle length). An ultrasound date is first converted to an equivalent LMP, then the same logic applies.

The fertile window

Since sperm can survive inside the body for up to 5 days, intercourse anywhere in the 5-day window before ovulation could lead to conception. The most fertile days are typically the last 3 days before ovulation plus the day of ovulation itself.

Accuracy

Conception dating is always an estimate. Ovulation timing varies, and fertilization can occur up to 24 hours after the egg is released. Use this as a reference, not a definitive answer.