Free · Historical first-trimester dating

First Trimester Crown Rump Dating Estimate

Evaluate the modified Robinson–Fleming crown-rump relationship for a measured first-trimester ultrasound length.

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Calculator inputs

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

Use the result with context

Health results are educational estimates, not a diagnosis or treatment recommendation. A qualified clinician can interpret measurements in the context of your health.

Formula and method

CRL 25 mm gives approximately 64.73 gestational days, about 9 weeks and 2 days when rounded to the nearest day.

Gestational days = 8.052 × √(1.037 × CRL in mm) + 23.73.

Worked example

Enter these known values and leave the other values blank.

Crown rump length
25 mm
Gestational age
9:2 wks / days

Assumptions and limitations

  • The BMUS professional guidance identifies the modified Robinson–Fleming best-fit expression including the 1.037 calibration factor. This is an ultrasound dating estimate, not a precise conception date. A qualified ultrasound measurement and local dating protocol are required.
  • This implementation limits interpretation to 6–14 gestational weeks (42–98 days), the interval stated in the cited primary validation population. The positive square-root branch applies; the algebraic squared inverse below that age interval must not be used. The gestational-age control remains output-only as captured.
  • Gestational age is counted on the obstetric dating scale, not from fertilization. The source’s time units use conventional month length where offered; months are approximate display conversions, while clinical gestation is normally expressed in weeks and days.

Common questions

Is the result an exact conception date?

No. The historical CRL relationship estimates obstetric gestational age from ultrasound measurement; it does not establish a precise conception date.

Can I enter gestational age as an input?

The captured interface presents age as an output-only estimate. The internal algebraic inverse is limited to the positive branch and is not an exposed measurement input.

References

Restrict this first-trimester dating estimate to 6–14 gestational weeks (42–98 days), the primary validation-study interval, and the principal positive square-root branch. This rejects the captured squared inverse below its meaningful gestational-age range.

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