crown rump length gestational age calculator

Estimating pregnancy dating in early gestation is one of the most common uses of obstetric ultrasound. The crown-rump length (CRL) measurement is considered the most reliable ultrasound marker for assigning gestational age in the first trimester. This calculator gives a quick estimate and can help you understand your scan report.

What is crown-rump length?

Crown-rump length is the measurement from the top of the embryo or fetus (the crown) to the bottom of the torso (the rump). It does not include the limbs or yolk sac. CRL is generally measured in millimeters during the first trimester, especially between about 6 and 13 weeks.

Because growth in early pregnancy follows a predictable pattern, CRL can be converted into gestational age with good accuracy. In routine care, CRL-based dating is often used to confirm or adjust expected due date calculations based on last menstrual period.

How this gestational age calculator works

Dating formula used

This tool uses a commonly cited first-trimester relationship:

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

The result is rounded to the nearest day, then displayed as weeks + days. If a scan date is entered, the tool also estimates:

• Estimated LMP date (scan date minus gestational age)
• Estimated due date (EDD) (LMP + 280 days)
• Estimated conception date (approximately LMP + 14 days)

Why early ultrasound dating matters

• Helps align prenatal milestones with the correct gestational week
• Improves timing for screening tests and anatomy scans
• Supports better interpretation of fetal growth later in pregnancy
• Reduces confusion when cycle length is irregular or LMP is uncertain

How to use the calculator

1. Enter the CRL number from your ultrasound report.
2. Select the correct unit (mm or cm).
3. Add the ultrasound date for full dating outputs.
4. Click Calculate Gestational Age.

The result panel will show estimated gestational age and date-based outputs when scan date is available.

Typical CRL dating context

CRL dating is usually most accurate in early pregnancy and tends to be less precise as pregnancy advances. In standard obstetric practice, first-trimester CRL often carries stronger weight than uncertain menstrual dating, but final clinical decisions should always follow local guidelines and your clinician’s judgment.

Factors that may affect measurement quality

• Fetal position during scan
• Image quality and machine settings
• Sonographer technique
• Very early scans where the embryo is small
• Multiple gestation or unusual anatomy

Frequently asked questions

Is CRL or LMP more accurate for due date?

When LMP is uncertain or cycles are irregular, first-trimester CRL is often more reliable for dating.

Can I use this after 14 weeks?

CRL is primarily a first-trimester dating tool. Later in pregnancy, other biometry measurements are generally used for assessment.

Why might my due date change after ultrasound?

If menstrual dating and ultrasound dating differ enough, clinicians may adjust the estimated due date to improve pregnancy management accuracy.