What happens in each trimester?

First trimester (weeks 1–12): rapid cell division, organ formation, and the highest risk of miscarriage. Second trimester (weeks 13–26): growth and movement begin, typically the most comfortable period. Third trimester (weeks 27–40): rapid growth, preparation for birth, increasing discomfort.

When does the second trimester start?

The second trimester begins at 13 weeks and 0 days of gestational age (counted from your LMP). Many people find this trimester more comfortable as early pregnancy symptoms often ease.

What is a full-term pregnancy?

A full-term pregnancy is defined as 39 weeks 0 days to 40 weeks 6 days. Early term is 37–38 weeks, late term is 41 weeks, and post-term is 42 weeks or more.

Does the calculator keep what I enter?

The values exist only in the open page and are not written to browser storage, so reloading the page clears them. All calculations run in your browser. Our servers are not involved at any point.

Pregnancy trimester calculator

Find out which trimester of pregnancy you are in right now. Enter your last menstrual period date or conception date to see your current trimester, gestational age in weeks and days, and your estimated due date. All calculations run locally in your browser. This is an estimate. Confirm with a healthcare provider.

Calculate from

First day of last period

Results are for informational purposes and do not constitute medical advice.

How it works

The trimester comes from completed weeks of gestational age: 0 to 12 weeks is the first trimester, 13 to 26 weeks the second, and 27 weeks onward the third. So 12 weeks and 6 days still counts as the first trimester, and 13 weeks and 0 days is the second. Weeks remaining are counted to the estimated due date and rounded up to whole weeks. All date arithmetic runs in your browser.

Processing runs in your browser

All calculations happen locally in your browser tab. Our servers are not involved at any point.

This tool provides an estimate only. Confirm your due date and gestational age with a qualified healthcare provider. Ultrasound measurements may result in a different estimate.

Technical specification

Naegele's rule dates a pregnancy from the first day of the last menstrual period and places the estimated due date 280 days (40 weeks) later. The American College of Obstetricians and Gynecologists describes this dating method, and its preference for early ultrasound when the two disagree, in Committee Opinion No. 700, Methods for Estimating the Due Date (2017).

Formula: LMP + 280 days; conception + 266 days
Trimesters: 0–12, 13–26, 27+ completed weeks
Date handling: Whole calendar days, local time zone
Browser API: JavaScript Date. No library

Related operations

To find an exact age from a past birthday, try the age calculator. For working out gaps between any two dates, use the date calculator. To work out a pre-pregnancy BMI from your usual height and weight, use the BMI calculator.

Frequently asked questions

What happens in each trimester?: First trimester (weeks 1–12): rapid cell division, organ formation, and the highest risk of miscarriage. Second trimester (weeks 13–26): growth and movement begin, typically the most comfortable period. Third trimester (weeks 27–40): rapid growth, preparation for birth, increasing discomfort.
When does the second trimester start?: The second trimester begins at 13 weeks and 0 days of gestational age (counted from your LMP). Many people find this trimester more comfortable as early pregnancy symptoms often ease.
What is a full-term pregnancy?: A full-term pregnancy is defined as 39 weeks 0 days to 40 weeks 6 days. Early term is 37–38 weeks, late term is 41 weeks, and post-term is 42 weeks or more.
Does the calculator keep what I enter?: The values exist only in the open page and are not written to browser storage, so reloading the page clears them. All calculations run in your browser. Our servers are not involved at any point.

Last reviewed September 30, 2026

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