Crown-rump length (CRL) to gestational age calculator

The measurement that dates a pregnancy

If you have had an early scan, there is a number on the report labelled CRL, in millimetres. It is the crown-rump length: the straight-line distance from the top of your baby's head to the bottom of the torso, measured with the fetus lying neutrally, neither curled nor stretched. Everything about your due date, from that scan onwards, comes from that one measurement.

It works because early embryos grow at an extraordinarily uniform rate. Before about 14 weeks, size really does tell you age — genetics and environment have not yet started pulling babies apart in size. After that the relationship weakens fast, which is why third-trimester scans are poor at dating and should not be used to move a due date on their own.

An equation from 1975 that is still right

In 1975, Hugh Robinson and John Fleming published a paper in the British Journal of Obstetrics and Gynaecology with the unpromising title A critical evaluation of sonar 'crown-rump length' measurements. What they had actually done was work out, carefully, where the errors in ultrasound measurement came from — beam width, velocity calibration, operator variation — and then fit a regression that accounted for them.

The equation they produced is still in daily clinical use half a century later. That is not sentiment. When the INTERGROWTH-21st Project built a modern international standard from thousands of pregnancies across eight countries and published it in 2014, it agreed with Robinson to within about a day across the range where both apply.

There is one wrinkle worth knowing about, because it explains why different calculators give answers a day apart. Robinson subtracted his own instrument correction — 1 mm plus 3.7% — from the measurements before fitting the curve. The equation as printed in the paper therefore expects corrected measurements. The form used clinically carries that 3.7% back into the formula as a × 1.037 inside the square root. This calculator uses the clinical form and shows you the other one too, rather than quietly picking one.

Reading the answer

The headline is a gestational age in weeks and days — 11 weeks and 2 days for a 45 mm measurement — and the same figure in the 11+2 notation your maternity notes use. Underneath it you get the exact figure before rounding, the uncorrected 1975 value, the INTERGROWTH-21st value, and a 95% range derived from INTERGROWTH's own published standard deviation.

That range is the honest part. Dating by crown-rump length is accurate to about five to seven days in the first trimester. It is the best method there is, and it is still a range.

Add the scan date and you also get the due date, calculated as the scan date plus the remainder of the 280 days.

Why the tool stops at 84 mm

This is the question the tool gets asked most, and the answer is written down in black and white. The NHS Fetal Anomaly Screening Programme handbook sets the eligibility for combined screening at a CRL “between 45.0mm and 84.0mm”, and is explicit about what happens beyond it: “if the CRL is greater than 84.0mm, the gestational age is calculated using the head circumference (HC)”.

So above 84 mm, crown-rump length is simply not the measurement in use any more — the fetus has grown too curved and too long for a reliable crown-to-rump line, and head circumference takes over. If your report shows a CRL above 84 mm, use the gestational age printed on the report itself.

At the other end, below about 10 mm the curve is so steep that a fraction of a millimetre shifts the answer by days, and the INTERGROWTH standard does not extend down there at all. That is why the two columns diverge by around five days at 10 mm: one of them is being extrapolated beyond its own published floor. Sonographers dating very early pregnancies know this and report accordingly.

Millimetres, not centimetres

Worth checking before anything else. Some reports print CRL in centimetres. 4.5 cm is 45 mm, and entering 4.5 instead of 45 will give an answer that is wrong by a month. This calculator watches for that and says so rather than producing a plausible-looking wrong number.

When a scan changes your due date — and when it does not

If a scan disagrees with the date you had, the scan usually wins, but not always. ACOG publishes thresholds: the due date is changed if the discrepancy is more than 5 days up to 8 weeks and 6 days, or more than 7 days from 9+0 to 13+6 weeks. Below those thresholds the original date stands, because the scan is not precise enough to justify moving it.

IVF pregnancies are the exception in the other direction. Because the date of fertilisation is a logged event rather than an estimate, IVF dates are not revised by a scan at all.

What this cannot do

It is one number from one measurement, and it cannot see the scan. It does not know whether the calipers were well placed, whether the fetus was curled, or whether the image was taken in a good plane — and all three change the measurement. It cannot handle twins, who are each measured separately and normally dated from the larger of the two crown-rump lengths.

Most importantly, it is not a substitute for the report. The gestational age printed on your scan report was produced by a machine calibrated for the job and read by a person who was looking at your pregnancy. If this calculator and your report disagree by a day, believe the report.