CRL Gestational Age & Redating Calculator
Enter the crown-rump length from a first-trimester ultrasound to estimate gestational age in weeks and days. If you add LMP dating, the calculator also shows the signed discrepancy, expected CRL, and the ACOG redating threshold.
CRL-based gestational age12 1/7 weeks
- CRL-based gestational age in days
- 85
- Difference from LMP dating
- 1 day; CRL dating is ahead of LMP dating.
- Expected CRL for LMP age (mm)
- 54.03
- ACOG redating threshold
- More than 7 days from 9 0/7 through 13 6/7 weeks
- Redating assessment
- Threshold not exceeded: 1 day is not more than the 7-day ACOG cutoff.
CRL dating uses the corrected Robinson-Fleming first-trimester equation and rounds the ultrasound age to the nearest day.
Do not apply the LMP redating assessment when dating was already established by IVF, embryo transfer, or another assisted-reproduction record.
The ACOG assessment is a first-trimester LMP-versus-ultrasound threshold check and uses a strict more-than rule.
How to use this calculator
- Enter the ultrasound crown-rump length in millimeters.
- Choose whether to compare the CRL-based age with LMP dating.
- If comparing, enter the completed LMP weeks and extra days on the scan date.
- Review the CRL age, day difference, expected CRL, threshold, and assessment.
How CRL gestational age is calculated
Crown-rump length, or CRL, is the embryo or fetus length from the top of the head to the rump on an early ultrasound. This calculator uses the corrected Robinson-Fleming first-trimester equation, with CRL entered in millimeters:
U = 8.052 x sqrt(1.037 x C) + 23.73
- C is the measured CRL in millimeters.
- U is the ultrasound gestational age in days.
The displayed ultrasound age is rounded to the nearest whole day, then converted to completed weeks plus remaining days. For example, 85 days is shown as 12 1/7 weeks. The calculator rejects CRL values below 5 mm or above 85 mm instead of extrapolating beyond the intended first-trimester range.
How the LMP comparison works
When LMP comparison is turned on, the entered LMP age is converted to days:
L = 7 x weeks + days
The signed discrepancy is:
D = round(U) - L
A positive result means CRL dating is ahead of LMP dating. A negative result means CRL dating is behind LMP dating. The expected CRL for the LMP age is the Robinson-Fleming equation solved backward:
E = ((L - 23.73) / 8.052)^2 / 1.037
This reverse value is useful when you want to see how far the measured CRL sits from the size implied by the LMP age.
The ACOG threshold used here
For first-trimester LMP-versus-ultrasound dating, this calculator applies the ACOG more-than thresholds from the entered LMP age: more than 5 days through 8 6/7 weeks, and more than 7 days from 9 0/7 through 13 6/7 weeks. A discrepancy exactly equal to 5 or 7 days does not exceed the threshold.
The related due-date redating calculator covers broader dating situations, including later ultrasound ages and whether the current EDD came from a reliable LMP, uncertain LMP, earlier ultrasound, or assisted reproduction. The Hadlock fetal-weight calculator is separate because fetal weight estimation uses later biometric measurements, not CRL.
What this calculator leaves out
The result does not judge scan quality, fetal position, caliper placement, cycle reliability, bleeding history, multiple gestation, embryo-transfer dating, or the full clinical record. CRL size alone cannot diagnose whether pregnancy development is normal, and ART or embryo-transfer dates should not be replaced by a mechanical LMP comparison.
Worked example
Suppose the ultrasound report lists a CRL of 55 mm, and the LMP age on the scan date is 12 0/7 weeks. The CRL equation gives 8.052 x sqrt(1.037 x 55) + 23.73 = 84.54 days, which rounds to 85 days, or 12 1/7 weeks.
The LMP age is 7 x 12 + 0 = 84 days, so the signed discrepancy is 85 - 84 = 1 day. CRL dating is 1 day ahead of LMP dating. Solving the equation backward for 84 LMP days gives an expected CRL of about 54.03 mm. Because 12 0/7 weeks is in the 9 0/7 through 13 6/7 band, the threshold is more than 7 days, so a 1-day difference does not exceed it.
Common questions
What does CRL mean on an ultrasound report?
CRL means crown-rump length. It is the measured length from the top of the embryo or fetus to the rump and is commonly used for first-trimester pregnancy dating.
How is gestational age calculated from CRL?
This calculator uses the corrected Robinson-Fleming equation: U = 8.052 x sqrt(1.037 x CRL) + 23.73, where U is gestational age in days and CRL is in millimeters. The result is rounded to the nearest day and displayed as weeks plus days.
Why can CRL dating differ from LMP dating?
LMP dating assumes ovulation timing from the menstrual period, but ovulation can occur earlier or later than expected. CRL dating depends on the ultrasound measurement itself, which can vary with fetal position, image quality, and caliper placement.
When does ACOG recommend changing a due date?
For the first trimester, ACOG uses a more-than threshold: more than 5 days before 9 0/7 weeks and more than 7 days from 9 0/7 through 13 6/7 weeks. A difference equal to the threshold does not meet the more-than rule.
Which date is used after IVF or embryo transfer?
Pregnancies dated by assisted reproduction use the embryo age and transfer date. A routine CRL-versus-LMP threshold should not replace ART-derived dating.
Does a small or large CRL mean something is wrong?
Not by itself. CRL is a dating measurement, and a single size comparison cannot diagnose fetal growth, viability, or pregnancy development. Clinicians interpret it with the ultrasound findings and the rest of the medical record.