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Crown-rump length: how far along is the pregnancy at the dating scan?

Convert the crown-rump length measured at an early ultrasound into gestational age with the Robinson formula, and get the due date and today's pregnancy age.

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Gestational age at the scan: weeks

11 weeks

Robinson formula, valid for a CRL of 10 to 84 mm

Estimated due date (40 weeks)

May 18, 2027

and days
1 days
Pregnancy start date (2 weeks after the last period)
August 25, 2026
Pregnancy age today: weeks
12 weeks
and days
1 days

This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.

Dating a pregnancy with an early scan

Early in pregnancy, embryos grow at almost exactly the same rate. Measuring the crown-rump length (CRL), from the top of the head to the bottom of the rump, on a first-trimester ultrasound therefore gives the most accurate estimate of gestational age and of the due date, more reliable than the date of the last period.

The formula

The calculator uses the formula published by Robinson and Fleming in 1975, still the most widely used:

Gestational age (days) = 8.052 × √CRL (mm) + 23.73

It is valid for a CRL between about 10 and 84 mm, roughly 7 to 14 weeks. After that, the head circumference or the biparietal diameter is used. Newer formulas, such as INTERGROWTH-21st, give results within a day or two.

Gestational age is counted from the first day of the last menstrual period, about two weeks before conception. The due date is 280 days, or 40 weeks, on that count.

Example

A scan one week ago measured a CRL of 45 mm:

  • Gestational age at the scan: 8.052 × √45 + 23.73 = 78 days, or 11 weeks and 1 day
  • Estimated due date: the scan date plus 202 days
  • Today, one week later: 12 weeks and 1 day

When does the scan change the due date?

ACOG and the Society for Maternal-Fetal Medicine advise using the ultrasound date when it differs from the date based on the last period by more than 5 days before 9 weeks, or more than 7 days between 9 and 14 weeks. Once set from an early scan, the due date should not be changed by later scans, because later differences reflect the baby’s growth rather than dating errors.

Good to know

  • The CRL is measured with the embryo in a neutral position, neither curled nor stretched, which is why the sonographer may take several measurements.
  • A due date is an estimate: most babies are born between 37 and 42 weeks, and only about 1 in 20 arrives on the day itself.
  • Your provider will confirm the dates and plan the next scans, such as the anatomy scan around 18 to 22 weeks.

Frequently asked questions

What is the crown-rump length?

The length of the embryo or fetus from the top of the head to the bottom of the rump, measured on an ultrasound scan in the first trimester. Because early growth varies little between babies, it is the most accurate way to date a pregnancy.

Which formula does the calculator use?

The Robinson and Fleming formula: gestational age in days = 8.052 × √(CRL in mm) + 23.73. It is valid for a CRL of about 10 to 84 mm, roughly 7 to 14 weeks. Newer formulas, such as INTERGROWTH-21st, give very similar results.

Does the scan date replace my period dates?

Usually yes, when the two differ by more than about 5 to 7 days before 14 weeks, according to ACOG guidance. Your provider decides which due date to keep, and it is not changed later by subsequent scans.

Why is the due date 40 weeks?

Gestational age is counted from the first day of the last period, about 2 weeks before conception, and an average pregnancy lasts about 280 days, or 40 weeks, on that count. Only about 1 baby in 20 arrives on the due date itself.

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Results are estimates provided for information purposes. They do not replace professional advice.

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