Which dating method is more accurate?

A first-trimester ultrasound CRL (10–84 mm) is the most accurate method per ACOG. LMP-based dating is acceptable when ultrasound is not available and the cycle is regular.

When should ultrasound dating replace LMP dating?

ACOG suggests redating the pregnancy when the ultrasound estimate differs from LMP by more than ~5 days in the first trimester or ~7 days in the early second trimester.

Why does the CRL formula stop at 84 mm?

Above ~84 mm CRL (≈ 14 weeks) the embryo is large enough that flexion affects the measurement; ACOG switches to biparietal diameter, head circumference, abdominal circumference, and femur length for later dating.

What is the estimated due date (EDD)?

EDD is 280 days after the LMP (Naegele’s rule) or 280 days after the LMP-equivalent date derived from the CRL. Most births happen within ±2 weeks of EDD.

Is my data stored anywhere?

No. The calculation runs entirely in your browser.

Gestational Age 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 gestational age calculator

Pick a dating method — “From LMP” or “From ultrasound CRL”.
For LMP mode, enter the first day of your last menstrual period.
For CRL mode, enter the scan date and the crown–rump length (mm); valid range is 10–84 mm.
Optionally pick a reference date — leave blank for today.
Press “Calculate gestational age” to see weeks + days and the estimated due date.
Use Copy to save the line or Reset to clear the form.

About this gestational age calculator

The gestational age calculator computes how far along a pregnancy is using one of the two methods recommended by ACOG’s pregnancy dating guidance (Committee Opinion 700, 2017; reaffirmed jointly with ASRM and SMFM). LMP mode uses Naegele’s rule — gestational age = today − LMP, EDD = LMP + 280 days — and assumes a 28-day cycle with ovulation on day 14. CRL mode uses the Robinson–Fleming formula (Robinson HP, Fleming JE. Br J Obstet Gynaecol 1975;82:702-10), which is the formula ACOG endorses for first-trimester ultrasound dating: GA in days = 8.052 × √(CRL × 1.037) + 23.73, valid for crown–rump lengths of 10 to 84 mm (approximately 7w 0d to 14w 0d).

In CRL mode the tool calculates GA at the scan date, back-derives the LMP-equivalent date, then projects forward to your reference date to get current GA and forward 280 days from the LMP-equivalent to get the EDD. Per ACOG, first-trimester ultrasound dating supersedes LMP when the two differ by more than 5–7 days, depending on the gestational age at scan.

Worked example: scan date June 1, CRL 60 mm ⇒ GA at scan ≈ 12w 3d (Robinson–Fleming), LMP-equivalent ≈ March 9, EDD ≈ December 14.

This tool is for general information using published dating formulas and is not a substitute for a clinician-confirmed dating ultrasound 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

Which dating method is more accurate?: A first-trimester ultrasound CRL (10–84 mm) is the most accurate method per ACOG. LMP-based dating is acceptable when ultrasound is not available and the cycle is regular.
When should ultrasound dating replace LMP dating?: ACOG suggests redating the pregnancy when the ultrasound estimate differs from LMP by more than ~5 days in the first trimester or ~7 days in the early second trimester.
Why does the CRL formula stop at 84 mm?: Above ~84 mm CRL (≈ 14 weeks) the embryo is large enough that flexion affects the measurement; ACOG switches to biparietal diameter, head circumference, abdominal circumference, and femur length for later dating.
What is the estimated due date (EDD)?: EDD is 280 days after the LMP (Naegele’s rule) or 280 days after the LMP-equivalent date derived from the CRL. Most births happen within ±2 weeks of EDD.
Is my data stored anywhere?: No. The calculation runs entirely in your browser.