Conception Calculator - Estimate Your Conception Date

Pregnancy Dating Tool

Estimate your conception date using your last menstrual period, cycle length, estimated due date, or ultrasound gestational age. See the estimated conception window and pregnancy timeline in one place.

Results are estimates for pregnancy dating, not a confirmation of the exact conception date.

Choose Your Calculation Method

Select the information you have available. Only the relevant calculator appears below.

LMP and Cycle Length Use the first day of your last menstrual period and average cycle length.

Estimated Due Date Work backward from an estimated due date to estimate conception.

Ultrasound Dating Use the ultrasound date and gestational age in weeks and days.

First Day of Last Period (LMP)

Enter the first day of your most recent menstrual period.

Cycle Length (days)

A 28-day cycle is used as the default. Adjust if your average cycle differs.

Estimated Due Date

Enter the estimated due date given by your healthcare provider or previous pregnancy dating calculation.

Ultrasound Date

Use the date when the gestational age was measured.

Gestational Age (weeks)

Enter the weeks reported by the ultrasound.

Additional Days

Enter the additional days reported with the gestational age.

Estimated Conception Date

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LMP Based

Estimated Due Date -

Estimated Pregnancy Start -

Estimated Gestational Age Today -

Estimated Days Pregnant -

Estimated Fertile/Conception Window -

Estimated 12-Week Date -

Estimated 20-Week Date -

Estimated 28-Week Date -

Estimated 40-Week Date -

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How the Conception Calculator Works

This calculator estimates a likely conception date by working backward or forward from pregnancy dating information. The calculation uses standard pregnancy dating intervals and should be treated as an estimate rather than an exact record of the day conception occurred.

LMP Dating

The calculator starts with the first day of the last menstrual period and adjusts the estimate according to the entered cycle length.

Due Date Dating

The calculator works backward from the estimated due date to produce an estimated conception date.

Ultrasound Dating

The calculator uses the reported gestational age to estimate the pregnancy start and conception timing.

LMP and Cycle Length

For a typical 28-day cycle, pregnancy dating commonly uses approximately 280 days from the first day of the last menstrual period to the estimated due date. A conception estimate is placed about 14 days after the beginning of a 28-day cycle. The calculator adjusts this estimate when a different cycle length is entered.

Estimated Due Date

If you already have an estimated due date, the calculator works backward by approximately 266 days to estimate conception. The result is an estimate because ovulation and conception do not necessarily occur on the same day for every pregnancy.

Ultrasound Method

When ultrasound dating provides a gestational age, the calculator works backward from the ultrasound date to estimate the pregnancy start and then estimates conception. Ultrasound-based dating depends on the gestational age reported from the scan. ACOG research and clinical guidance recognize ultrasound as an important method for estimating gestational age.

Important: This calculator provides an estimated conception date for informational purposes. It cannot determine the exact date of conception, ovulation, intercourse or fertilization. Pregnancy dating should be confirmed with your healthcare provider, especially when LMP dates are uncertain or ultrasound measurements differ.

Conception Date Calculation Methods

Method	Information Required	Basic Calculation
LMP and Cycle Length	First day of LMP and average cycle length	Estimated LMP + cycle-adjusted conception timing
Estimated Due Date	Estimated due date	Due date minus approximately 266 days
Ultrasound	Ultrasound date and gestational age	Ultrasound date minus gestational age, then estimate conception

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Due Date Calculator Date Calculator Age Calculator

Frequently Asked Questions

No. The calculator estimates a likely date based on pregnancy dating information. Ovulation and fertilization timing can vary, so the result should not be treated as an exact conception record.

A 28-day cycle is commonly used as the standard reference. A longer or shorter average cycle can shift the estimated ovulation and conception timing, so the calculator allows you to enter your typical cycle length.

Select the Estimated Due Date method and enter the due date. The calculator will work backward to estimate the conception date.

Yes. If an ultrasound provides a gestational age, you can enter the ultrasound date and the reported weeks and days. The calculator then works backward to estimate conception timing.

Different calculators can use different assumptions for cycle length, ovulation timing, pregnancy dating and ultrasound measurements. Small differences are expected when the underlying inputs differ.

Use the method that matches the most reliable information you have. If a healthcare provider has established pregnancy dating from an ultrasound, use the reported gestational age and ultrasound date rather than guessing an LMP.

References

American College of Obstetricians and Gynecologists (ACOG), Methods for Estimating the Due Date, Committee Opinion No. 700.

American College of Obstetricians and Gynecologists (ACOG), Estimating Gestational Age From Ultrasound, Obstetrics and Gynecology.

American College of Obstetricians and Gynecologists (ACOG), Practice Bulletin: Methods for Estimating the Due Date.