How this calculator works
This page uses the 2014 INTERGROWTH-21st CRL dating equation over its 15 to 95 mm tabulated range. CRL must come from a correctly obtained ultrasound measurement. The standard deviation describes model dating uncertainty and is not measurement precision. Reverse mode inverts the dating equation; it is not the separate fetal-size-for-age growth equation and should not be used to assess growth. Later pregnancies require other ultrasound parameters. The result is an estimate for information only and is not medical advice.
Formula
GA days = 40.9041 + 3.21585 × sqrt(CRL mm) + 0.348956 × CRL mm. SD days = 2.39102 + 0.0193474 × CRL mm.
Worked example
At CRL 25 mm the equation gives 65.70725 days, approximately 9 weeks 3 days. At 64 mm it gives 88.964084 days, approximately 12 weeks 5 days.
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Frequently asked questions
Is inverse mode a growth chart?
No. Inverting a regression for dating does not produce the separate regression for expected size at a given age.
Why are very small CRLs rejected?
The page restricts the equation to its published dating-table range rather than extrapolating.
Can this diagnose a problem?
No. Measurement technique, prior dating and the clinical assessment all matter.