Where do the trimester boundaries come from?

ACOG dating guidance: T1 ends at 13w 6d (end of the 13th completed week), T2 runs 14w 0d–27w 6d, and T3 begins at 28w 0d.

Why isn’t the third trimester just “7 to 9 months”?

Pregnancy is counted in completed gestational weeks, not calendar months. Months vary in length and don’t map cleanly to the 280-day pregnancy count.

What changes between trimesters?

Major clinical milestones cluster by trimester: organogenesis and dating scans in T1, the anatomy scan and quickening in T2, growth scans and term planning in T3.

How accurate is LMP-based trimester dating?

It depends on the regularity of your cycle. ACOG considers a first-trimester ultrasound (especially with a CRL of 10–84 mm) the most accurate dating method, and clinicians will reset trimester boundaries to match it.

Is my data stored anywhere?

No. The calculator runs entirely in your browser.

Trimester Calculator

Written by Golam Rabbani, Founder & Lead Engineer — Last updated 2026-05-01

This interactive tool requires JavaScript. Read the formula and worked example below; you can compute the result by hand.

How to use this trimester calculator

Select the first day of your last menstrual period (LMP).
Optionally pick a different reference date — otherwise the tool uses today.
Press “Find trimester” to see your current trimester and the calendar dates for each one.
Use Copy to save the result or Reset to clear the form.

About this trimester calculator

The trimester calculator finds which trimester of pregnancy you are in on a given date and shows the calendar boundaries for each trimester based on your LMP. The boundaries follow ACOG dating guidance (Committee Opinion 700, 2017): the first trimester is from the LMP through 13w 6d, the second trimester is 14w 0d through 27w 6d, and the third trimester runs from 28w 0d to delivery (estimated at 40w 0d).

It computes gestational age the same way clinicians do — total days from LMP to the reference date, expressed as weeks + days — then maps that age into one of the three trimesters and reports the start/end calendar date of each. You can use it to plan an anatomy scan around the start of the second trimester (~20w) or a glucose tolerance test in the late second / early third trimester, but the dates are an estimate, not a clinical schedule.

Worked example: LMP June 1; on November 15 you are 23w 5d — second trimester. Second trimester started ~September 7 (14w 0d) and ends ~December 21 (27w 6d).

Pregnancy dating from LMP assumes a 28-day cycle; first-trimester ultrasound is more accurate per ACOG, and this tool is not a substitute for clinician confirmation by a qualified clinician. This calculator is for general educational purposes only and is not medical advice — consult a licensed healthcare professional for personal guidance.

FAQ

Where do the trimester boundaries come from?: ACOG dating guidance: T1 ends at 13w 6d (end of the 13th completed week), T2 runs 14w 0d–27w 6d, and T3 begins at 28w 0d.
Why isn’t the third trimester just “7 to 9 months”?: Pregnancy is counted in completed gestational weeks, not calendar months. Months vary in length and don’t map cleanly to the 280-day pregnancy count.
What changes between trimesters?: Major clinical milestones cluster by trimester: organogenesis and dating scans in T1, the anatomy scan and quickening in T2, growth scans and term planning in T3.
How accurate is LMP-based trimester dating?: It depends on the regularity of your cycle. ACOG considers a first-trimester ultrasound (especially with a CRL of 10–84 mm) the most accurate dating method, and clinicians will reset trimester boundaries to match it.
Is my data stored anywhere?: No. The calculator runs entirely in your browser.