A tool to support the clinical decisions of doctors and health professionals, not for self-diagnosis or self-treatment.
This calculator is inside the patient's file in Clinicato: open it from "Medical tools" while you chart the visit, it fills in from the patient's details and vital signs where they are recorded, and adds its result to the visit's plan in one tap, with the "Medical tools" add-on. Try it in your clinic
Calculator
How it works and when to use it
The calculator uses the Robinson and Fleming formula: gestational age in days = 8.052 × √CRL in mm + 23.73, then the due date from the scan date.
With the LMP it compares both dates using ACOG's redating thresholds: more than 5 days up to 8 weeks 6 days, more than 7 days from 9 weeks to 13 weeks 6 days.
A first-trimester CRL (up to 13 weeks 6 days) is the most accurate way to date a pregnancy.
Reference: Robinson HP, Fleming JE. Br J Obstet Gynaecol 1975;82:702-10 (GA days = 8.052 × √CRL + 23.73); redating thresholds: ACOG Committee Opinion No. 700 (2017)
Frequently asked questions
When is CRL the measurement to use?
In the first trimester up to about 84 mm (about 14 weeks), after which other measurements such as head circumference are used.
If the ultrasound and the LMP disagree, which date do I keep?
The LMP date stays when the difference is within the threshold for the gestational age at the scan, otherwise the ultrasound date is used, and the calculator shows the threshold and the decision.
Is the due date recalculated at every later scan?
No, once an accurate early scan has set it, later scans do not change it, since late differences in size may mean a growth problem rather than a dating error.
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Clinic software for this specialty
Try Clinicato for your clinic
These calculators are free here for everyone, and in Clinicato 49 medical tools work inside the patient's file with the "Medical tools" add-on: filled from the visit and the vital signs, with the result added to the visit's plan, next to appointments, prescriptions and invoices.