Why is pregnancy dated from the LMP, not from conception?

Because conception is rarely known precisely, but the date of the last period usually is. Clinicians count from LMP to keep dating consistent; “40 weeks” of pregnancy is really about 38 weeks from conception.

When do the trimesters start and end?

First trimester: LMP to 13w 6d. Second trimester: 14w 0d to 27w 6d. Third trimester: 28w 0d onward. This follows ACOG dating guidance.

How accurate is Naegele’s rule for the due date?

It’s a good first estimate. Only ~5% of babies actually arrive on the EDD; most births happen between 37 and 42 weeks. A first-trimester ultrasound CRL measurement is the most accurate dating method.

What if my cycles aren’t 28 days?

LMP-based dating assumes a 28-day cycle with ovulation on day 14. If your cycle is much longer or shorter, your clinician may adjust the EDD using ultrasound or known conception timing.

Is my information stored?

No. The calculation runs entirely in your browser.

Pregnancy Week 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 pregnancy week calculator

Select the first day of your last menstrual period (LMP).
Optionally pick a reference date — leave blank to use today.
Press “Calculate week” to see how many weeks + days pregnant you are, your trimester, and your estimated due date.
Use Copy to save the result line or Reset to clear the form.

About this pregnancy week calculator

The pregnancy week calculator counts gestational age in completed weeks and days from the first day of your last menstrual period (LMP) to a reference date, plus it returns the estimated due date (EDD) using Naegele’s rule of LMP + 280 days. Gestational age is the standard way pregnancy is described in clinic notes: a fetus described as 24w 3d is twenty-four weeks and three days past the LMP.

The math is straightforward: total days = reference date − LMP; weeks = floor(total days / 7); extra days = total days mod 7. The trimester boundaries follow ACOG dating guidance — first trimester ends at 13w 6d, second trimester runs 14w 0d to 27w 6d, and third trimester begins at 28w 0d. The estimated due date is LMP + 280 days; subtracting the reference date from that gives the days-remaining count.

Worked example: LMP January 1, today June 1 — that is 151 days, or 21 weeks + 4 days. Trimester: second. EDD: October 8 (280 days from LMP).

LMP-based dating assumes a 28-day cycle and ovulation on day 14. It is not a substitute for an ultrasound dating scan or 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

Why is pregnancy dated from the LMP, not from conception?: Because conception is rarely known precisely, but the date of the last period usually is. Clinicians count from LMP to keep dating consistent; “40 weeks” of pregnancy is really about 38 weeks from conception.
When do the trimesters start and end?: First trimester: LMP to 13w 6d. Second trimester: 14w 0d to 27w 6d. Third trimester: 28w 0d onward. This follows ACOG dating guidance.
How accurate is Naegele’s rule for the due date?: It’s a good first estimate. Only ~5% of babies actually arrive on the EDD; most births happen between 37 and 42 weeks. A first-trimester ultrasound CRL measurement is the most accurate dating method.
What if my cycles aren’t 28 days?: LMP-based dating assumes a 28-day cycle with ovulation on day 14. If your cycle is much longer or shorter, your clinician may adjust the EDD using ultrasound or known conception timing.
Is my information stored?: No. The calculation runs entirely in your browser.