What is the difference between LMP and conception date?

LMP (Last Menstrual Period) is the first day of your last period. Conception typically occurs about 14 days later. If calculating from conception, 266 days are added instead of 280.

Can my due date change?

Yes. An ultrasound in the first trimester may adjust your due date if measurements differ significantly from the LMP-based estimate.

What if I don't know my last period date?

An early ultrasound (before 12 weeks) can estimate gestational age accurately. Your doctor can calculate the due date from the ultrasound measurements.

Is 40 weeks exactly 9 months?

Not quite. 40 weeks equals about 9 months and 7 days. Pregnancy is counted from the LMP, which is roughly 2 weeks before conception.

Due Date Calculator

Calculate your baby's due date from last menstrual period or conception date. See trimester milestones.

Calculate From

Month

Day

Year

Estimated Due Date

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Weeks Remaining —

2nd Trimester Starts —

3rd Trimester Starts —

Extended More scenarios, charts & detailed breakdown ▾

Last Period Month

Day

Year

Due Date

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Weeks Remaining —

2nd Trimester —

3rd Trimester —

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Calculate From

Month

Day

Year

Cycle Length ?

days

Pregnancy Type

Due Date Estimates

Estimated Due Date —

Cycle-Adjusted Due Date —

Est. Conception Date —

Current Status

Current Week —

Trimester —

Days to Due Date —

Key Dates

2nd Trimester —

3rd Trimester —

Full Term (37 wks) —

Anatomy Scan (~20 wks) —

How to Use This Calculator

Choose whether to calculate from your last menstrual period or conception date, then enter the date. The calculator shows your due date, weeks remaining, and when each trimester begins.

Formula

From LMP: Due Date = LMP + 280 days
From Conception: Due Date = Conception + 266 days

Example

Example: LMP of March 1, 2026: Due date = December 6, 2026. 2nd trimester starts June 1, 3rd trimester starts August 29.

Frequently Asked Questions

LMP (Last Menstrual Period) is the first day of your last period. Conception typically occurs about 14 days later. If calculating from conception, 266 days are added instead of 280.
Yes. An ultrasound in the first trimester may adjust your due date if measurements differ significantly from the LMP-based estimate.
An early ultrasound (before 12 weeks) can estimate gestational age accurately. Your doctor can calculate the due date from the ultrasound measurements.
Not quite. 40 weeks equals about 9 months and 7 days. Pregnancy is counted from the LMP, which is roughly 2 weeks before conception.

Related Calculators

Sources & References (5) ▾

ACOG — Committee Opinion No. 700: Methods for Estimating the Due Date (2017) — American College of Obstetricians and Gynecologists
Tunón K et al. — A comparison between ultrasound and a reliable last menstrual period as predictors of the day of delivery in 15,000 examinations. Ultrasound Obstet Gynecol. 1996;8(3):178-185 — Ultrasound in Obstetrics & Gynecology
Mongelli M et al. — Influence of conception characteristics on the accuracy of ultrasound estimates of gestational age. Am J Obstet Gynecol. 2001;184(1):17-20 — AJOG
CDC — Pregnancy: Due Date Calculator Background — CDC
NHS — How your due date is calculated — NHS