How is pregnancy dated?

From the first day of the last menstrual period (LMP). Day 1 of pregnancy is LMP day, even though conception happens about 2 weeks later.

How long is full-term pregnancy?

37 to 42 weeks from LMP. The estimated due date is 40 weeks from LMP. Babies born before 37 weeks are preterm; after 42 weeks are post-term.

Why does dating start before conception?

LMP is observable and recordable. Actual conception happens roughly 14 days after LMP but is harder to pin down. Doctors use LMP for consistency.

How accurate is LMP-based dating?

Within about a week for most pregnancies. Less accurate for irregular cycles or unknown LMP. Early ultrasound is more precise.

When do trimesters start and end?

First: weeks 1-12. Second: weeks 13-26. Third: weeks 27-40. Some sources put the second/third boundary at week 28 instead.

Weeks Pregnant Calculator

First Day of Last Period (LMP)

The first day of your last menstrual period

Today's Date

Formula

Gestational age = today - LMP date; Due date = LMP + 280 days

Standard obstetric dating uses the first day of the last menstrual period (LMP) as week 0. Pregnancy is counted as 40 weeks (280 days) from LMP. Gestational age is the number of days since LMP, displayed as weeks and days. Trimester boundaries are at 13 weeks and 27 weeks.

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TL;DR

Enter your last menstrual period date. See gestational age, trimester, due date, and weeks remaining.

Enter the first day of your last period to see exactly how many weeks pregnant you are today. The calculator reports gestational age in weeks and days, identifies your trimester, and shows the estimated due date plus how many weeks remain until delivery.

Obstetric dating is counted from the first day of the last menstrual period, not from conception. That convention gives pregnancy a standard 40-week timeline even though actual fertilization happens about 2 weeks after LMP. This calculator gives you the exact week and day so you can track milestones and appointments.

You came here because

Common situations

Tracking prenatal appointments: Most doctor visits are timed by gestational age (12-week ultrasound, 20-week scan).
Following weekly development: Pregnancy apps and books describe development week by week. Knowing your exact week unlocks the right milestones.
Understanding test windows: Genetic tests, glucose screening, and other lab work have specific weeks they are most accurate.
Planning maternity leave: Calculate when your due date falls in the work calendar for leave-request timing.

Under the hood

How the calculation works

1Enter the first day of your last menstrual period (LMP).
2Confirm or change today's date.
3The calculator counts days since LMP.
4Days are converted to weeks plus extra days.
5Trimester is determined by week number.
6Due date is LMP plus 280 days.

Show me

A real example

Example: LMP February 15, 2026, today is May 24, 2026

1Days since LMP: 98 days
298 / 7 = 14 weeks exactly
3Trimester: second (13 to 27 weeks)
4Due date: February 15 + 280 days = November 22, 2026

Result: 14 weeks pregnant, second trimester, due November 22, 2026.

Watch out for

What can go wrong

Counting from conception instead of LMP: Doctors count from LMP. Counting from suspected conception gives a number about 2 weeks lower than the medical convention.
Using cycle day instead of period start: LMP is the first day of bleeding, not ovulation day. Even if you know the conception date, dating still uses LMP.
Treating the due date as exact: Only about 4% of babies arrive on their estimated due date. The due date is a target window. Full-term is anywhere from 37 to 42 weeks.
Mismatched LMP and ultrasound dates: If a first-trimester ultrasound dates the pregnancy more than 5 days off from LMP dating, the ultrasound date takes priority. The calculator only uses LMP.

Glossary

Related concepts

Term	Definition
Last menstrual period (LMP)	The first day of the most recent menstrual period before conception. The standard reference for pregnancy dating.
Gestational age	The age of the pregnancy in weeks and days, counted from LMP. Includes the two weeks before actual conception.
Trimester	A roughly three-month division of pregnancy. First: weeks 1-12. Second: weeks 13-26. Third: weeks 27-40.
Naegele's rule	A traditional method for estimating the due date: add 7 days, subtract 3 months, and add a year to the LMP date. Equivalent to LMP plus 280 days.

Make it better

Pro tips

Confirm with an early ultrasound: A first-trimester ultrasound (between 8 and 12 weeks) gives the most accurate dating. After 13 weeks, fetal growth variation makes dating less precise.
Track week-by-week development: Bookmark the current week in a pregnancy reference. The body and baby change quickly enough that knowing the right week unlocks the right information.
Schedule tests in their accuracy window: NIPT is most accurate after week 10. Glucose tolerance is standard at week 24-28. Asking too early or too late can mean repeat tests.
Use this with the ovulation calculator: If you are planning a pregnancy, the ovulation calculator predicts the fertile window. After conception, switch to this one for dating.

Common questions

Frequently asked questions

Quick reference

Pregnancy Milestones by Week

Key markers in a typical 40-week pregnancy

Week	Stage	Milestone
4	First trimester	Missed period; positive home pregnancy test
8	First trimester	First prenatal visit, first ultrasound common
12	First trimester	Reduced miscarriage risk; nuchal scan window
20	Second trimester	Anatomy scan, often gender reveal
28	Third trimester	Glucose tolerance test, Rh shot if needed
37	Third trimester	Considered full-term
40typical	Third trimester	Estimated due date

For related calculations, try the Pregnancy Calculator, or Date Difference. Browse all Calculator Online calculators for the full catalog.

Methodology

This calculator uses the standard weeks pregnant calculator formula. Results match those from established financial, scientific, and health references.

Reviewed by

Calculator Online Editorial Team. All formulas verified against authoritative sources before publication.

Last updated

2026-05-24