How is a due date calculated from LMP?

Naegele's Rule adds 280 days (40 weeks) to the first day of your last menstrual period. If your cycle is not 28 days, the due date shifts by the difference from 28; a 35-day cycle pushes the due date 7 days later.

How accurate is the LMP method?

The LMP method is accurate within 1-2 weeks for most people. An early ultrasound (before 14 weeks) is the most reliable way to confirm your due date because it directly measures fetal size.

What are the trimester dates?

The first trimester covers weeks 1-13, the second covers weeks 14-27, and the third covers weeks 28-40+. Full term is 39-40 weeks; preterm is before 37 weeks; post-term is after 42 weeks.

What is the difference between gestational age and fetal age?

Gestational age counts from the first day of your last period, which is typically 2 weeks before conception. Fetal age counts from conception. Doctors use gestational age by convention, so a pregnancy described as 8 weeks along has a fetus that is approximately 6 weeks old.

Pregnancy Due Date Calculator

Calculate your estimated due date and current gestational age using LMP, conception date, or ultrasound -- with trimester timeline and key milestone dates.

Calculation method

First day of last menstrual periodAverage cycle length (days)

Due Date

Feb 21, 2027

Gestational Age

8w 0d

56 days total

Days Remaining

224

~32 weeks

Trimester progress

1st trimester2nd trimester3rd trimester

Currently in trimester 1

Key milestones

Week 6--Heartbeat detectableJun 28, 2026
Week 10--Nuchal translucency windowJul 26, 2026
Week 13--End of first trimesterAug 16, 2026
Week 20--Anatomy scanOct 4, 2026
Week 24--Viability thresholdNov 1, 2026
Week 28--Third trimesterNov 29, 2026
Week 37--Full termJan 31, 2027
Week 40--Due dateFeb 21, 2027
Week 42--Post-term thresholdMar 7, 2027

This calculator uses standard obstetric formulas. Always confirm your due date with your healthcare provider.

How this tool works

This calculator estimates your pregnancy due date using one of three methods. The LMP method applies Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period and adjust for cycle length. If your cycle is longer or shorter than 28 days, the due date shifts by the difference. The conception date method adds 266 days (38 weeks) to the estimated date of conception. The ultrasound method calculates current gestational age from the age measured at a scan plus the days elapsed since the scan, then derives the due date from 280 minus that gestational age. All three methods converge on the same 40-week pregnancy timeline and produce gestational age, trimester position, and key milestone dates.

Worked example

If your last period started on April 20, 2026 with a standard 28-day cycle, adding 280 days gives a due date of January 25, 2027. On June 15, 2026 you would be 8 weeks 0 days pregnant, in the first trimester. With a 35-day cycle, the due date shifts 7 days later to February 1, 2027.

Frequently asked questions

How is a due date calculated from LMP?
Naegele's Rule adds 280 days (40 weeks) to the first day of your last menstrual period. If your cycle is not 28 days, the due date shifts by the difference from 28; a 35-day cycle pushes the due date 7 days later.
How accurate is the LMP method?
The LMP method is accurate within 1-2 weeks for most people. An early ultrasound (before 14 weeks) is the most reliable way to confirm your due date because it directly measures fetal size.
What are the trimester dates?
The first trimester covers weeks 1-13, the second covers weeks 14-27, and the third covers weeks 28-40+. Full term is 39-40 weeks; preterm is before 37 weeks; post-term is after 42 weeks.
What is the difference between gestational age and fetal age?
Gestational age counts from the first day of your last period, which is typically 2 weeks before conception. Fetal age counts from conception. Doctors use gestational age by convention, so a pregnancy described as 8 weeks along has a fetus that is approximately 6 weeks old.