Gestational Age from Crown-Rump Length
Gestational Age from Crown-Rump Length
Robinson and Fleming’s crown-rump length equation, in both of the two forms that circulate under that name, with its published accuracy. Refuses outside the 5 to 84 mm window it was derived in rather than extrapolating.
Crown-rump length to gestational age
Robinson and Fleming 1975Crown-rump length 60 mm, with the 1.037 calibration factor
Formula
with CRL in millimetres, for CRL between 5 and 84 mm
- 1.037
- Robinson’s own calibration correction, and the single point on which the two circulating forms of this equation differ. The 1975 paper reports a systematic overestimate of 1 mm from beam width and of 3.7% from oscilloscope scale factor and velocity calibration. NHS Scotland’s screening dating tables carry the factor and call this the current BMUS 2009 dating formula; LOINC 11914-9 codes the equation without it
- 5 to 84 mm
- the window outside which this page returns nothing. The regression was fitted to 334 measurements over 6 to 14 weeks of menstrual age, and 5 mm and 84 mm are where the equation itself puts those two ages. The 84 mm ceiling is also set independently: ACOG Committee Opinion 700 uses crown-rump length through 13 weeks 6 days, NHS Scotland switches to head circumference at 84.1 mm, and GLOWM says head circumference should be used above 84 mm
- plus or minus 4.7 days
- Robinson and Fleming’s stated accuracy for gestational age from a SINGLE crown-rump length, at 95% probability. From the mean of three measurements it is plus or minus 2.7 days. ISUOG’s biometry teaching gives first-trimester dating as plus or minus 5 days in 95% of cases, which is the same figure from a different direction
- other equations exist
- Robinson’s is the most widely used but not the only one. Pexsters and colleagues fitted a new curve to 3,064 scans in 2,597 pregnancies and note that Robinson’s came from 214 transabdominal scans in 80 women on mostly static-image equipment, and that MacGregor and colleagues found it generally underestimates gestational age. Hadlock also published a crown-rump length dating equation. They are not interchangeable
Worked example
Crown-rump length 60 mm, with the 1.037 calibration factor
60 × 1.037 = 62.22
√62.22 = 7.8880
8.052 × 7.8880 = 63.515, plus 23.73 = 87.2 days, which is 12w3d on the truncating obstetric convention
Days to the 280-day estimate = 280 − 87.2 = 193 days
On the form WITHOUT the 1.037 factor the same 60 mm reads 86.1 days (12w2d), 1.14 days younger — switch the selector to see it
Robinson's own accuracy puts the 95% interval on this answer at about plus or minus 4.7 days from one measurement, so the decimal place is arithmetic rather than precision
Enter 84.1 mm and the page returns nothing: that is above the window the equation was derived in and above the 84 mm ceiling at which dating switches to head circumference. Enter 4.9 mm and it returns nothing for the same reason at the other end
The two circulating forms of Robinson’s equation, side by side
| CRL (mm) | With 1.037 (days) | Without it (days) | Difference (days) |
|---|---|---|---|
| 5 | 42.1 (6w0d) | 41.7 (5w6d) | 0.33 |
| 10 | 49.7 (7w0d) | 49.2 (7w0d) | 0.47 |
| 20 | 60.4 (8w4d) | 59.7 (8w3d) | 0.66 |
| 30 | 68.6 (9w5d) | 67.8 (9w4d) | 0.81 |
| 45 | 78.7 (11w1d) | 77.7 (11w0d) | 0.99 |
| 60 | 87.2 (12w3d) | 86.1 (12w2d) | 1.14 |
| 70 | 92.3 (13w1d) | 91.1 (13w0d) | 1.23 |
| 84 | 98.9 (14w0d) | 97.5 (13w6d) | 1.35 |
What the equation rests on, and where it stops
| Property | Value | Source |
|---|---|---|
| Derivation sample | 334 measurements, 6 to 14 weeks of menstrual age | Robinson and Fleming 1975 |
| Accuracy, one measurement | plus or minus 4.7 days at 95% probability | Robinson and Fleming 1975 |
| Accuracy, mean of three | plus or minus 2.7 days at 95% probability | Robinson and Fleming 1975 |
| Measurement reproducibility | 1.2 mm average standard deviation across three readings | Robinson and Fleming 1975 |
| Sparse data | few points before 7 weeks or after 13 weeks | Pexsters et al. 2010 |
| Usable range | about 45 to 84 mm | GLOWM biometry chapter |
| Upper ceiling | 84 mm, then head circumference | ACOG CO 700; NHS Scotland dating tables; GLOWM |
| This page refuses | below 5 mm and above 84 mm | the derivation window and the 84 mm ceiling |
One equation, two published forms, and a window it must not be used outside
A first-trimester crown-rump length is the most accurate way to date a pregnancy, and Robinson and Fleming’s 1975 equation is the one most of the world’s dating charts are built on. The paper is a study of measurement error rather than of growth: it identifies the random sources and the systematic ones, quantifies both, and only then fits the curve. Three readings per fetus across 30 experiments gave an average standard deviation of 1.2 mm; the raw measurements overestimated by 1 mm from beam width and by 3.7% from calibration. Gestational age from a single corrected measurement came out at plus or minus 4.7 days at 95% probability, and plus or minus 2.7 days from the mean of three.
That 3.7% is why there are two equations. One published form multiplies the crown-rump length by 1.037 inside the square root and one does not, and both are attributed to the same 1975 paper. NHS Scotland’s screening dating tables carry the factor and name it the current BMUS 2009 dating formula; LOINC’s coded version of ‘GA from CRL, Robinson 75’ omits it. The gap runs from about a third of a day at 5 mm to about 1.35 days at 84 mm. That is small against the equation’s own error, but it is systematic: it shifts every pregnancy in a programme the same way, so a cohort dated on one form is not directly comparable with a cohort dated on the other. Both are offered above, and the page prints which was used.
The window matters more than the form. This page returns nothing below 5 mm or above 84 mm, and the refusal is deliberate: a square-root curve extrapolated past its data returns a confident, plausible, wrong gestational age, and there is nothing in the output to say so. The 84 mm ceiling is set three times over — ACOG Committee Opinion 700 uses crown-rump length through 13 weeks 6 days, NHS Scotland switches to head circumference at 84.1 mm, and the GLOWM biometry chapter says head circumference should be used above 84 mm — because the fetus curls and the greatest length stops being a crown-to-rump length at all. Below 5 mm the embryo is beneath the bottom of Robinson’s own 6-week data. Once a gestational age is established here it should be established ONCE: see the redating criteria for what ACOG allows a later scan to change, and the menstrual-interval calculator for the method this one supersedes. The screening biochemistry of pregnancy lives in this site’s reproductive, fertility and pregnancy laboratory category: that category owns the assay, this one owns the pregnancy. Those pages need a gestational age and none establishes one, which is what the dating pages here are for. An obstetric measurement is read alongside the whole clinical picture — history, serial trend, the other biometry — and never instead of it. It supports a clinician’s judgement rather than replacing it.
Frequently asked questions
Which Robinson formula should I use?
Whichever your unit’s charts use, and record which. The form with the 1.037 calibration factor is the one in NHS screening dating tables, described there as the current BMUS 2009 dating formula; the form without it is what LOINC codes as Robinson 1975. They differ by a third of a day at 5 mm and about 1.35 days at 84 mm — less than the equation’s own accuracy, but in a consistent direction.
Why does the page refuse above 84 mm?
Because dating switches to head circumference there, and the equation was not fitted beyond it. ACOG Committee Opinion 700 makes crown-rump length the measurement of choice through 13 weeks 6 days; NHS Scotland’s dating tables switch at 84.1 mm; the GLOWM biometry chapter says head circumference should be used above 84 mm. Extrapolating would return a plausible number with nothing to flag it.
How accurate is first-trimester dating?
Robinson and Fleming’s own figures are plus or minus 4.7 days at 95% probability from one measurement and plus or minus 2.7 days from the mean of three. ISUOG’s biometry teaching quotes the estimated due date as plus or minus 5 days in 95% of cases. That is the most accurate dating available at any point in pregnancy.
Are there other crown-rump length equations?
Yes, and they disagree. Hadlock published one; Pexsters and colleagues fitted a new curve in 2010 to 3,064 scans, noting that Robinson’s came from 214 transabdominal scans in 80 women on mostly static-image equipment and that MacGregor and colleagues found it generally underestimates gestational age.
Does this give a due date?
It gives the gestational age in days and the days remaining to the 280-day estimate. It cannot give a date: the expression language behind this site is numeric only and has no date type. Add the days remaining to today on any calendar.
Related calculators
References
- Robinson HP, Fleming JEE. A critical evaluation of sonar crown-rump length measurements. Br J Obstet Gynaecol. 1975;82(9):702-10.
- NHS Scotland Pregnancy and Newborn Screening Programme. Crown Rump Length and Head Circumference Dating Tables. Edinburgh: Public Health Scotland.
- Regenstrief Institute. LOINC 11914-9: Gestational age estimated from crown rump length on US by Robinson 1975 method. Indianapolis: Regenstrief Institute.
- Pexsters A, Daemen A, Bottomley C, et al. New crown-rump length curve based on over 3500 pregnancies. Ultrasound Obstet Gynecol. 2010;35(6):650-5.
- American College of Obstetricians and Gynecologists. Committee Opinion No. 700: Methods for Estimating the Due Date. Obstet Gynecol. 2017;129(5):e150-4.
- Global Library of Women’s Medicine. Ultrasound Biometry: Pregnancy Dating and Assessment of Fetal Size and Growth. London: GLOWM.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/