Crown-Rump Length Gestational Age Calculator

About the Crown-Rump Length Gestational Age Calculator

The Crown-Rump Length (CRL) Gestational Age Calculator converts a first-trimester ultrasound CRL measurement into gestational age and estimated due date using validated formulas. CRL is the single most accurate method for establishing gestational age in early pregnancy, with a margin of error of only ±3–5 days when measured between 7 and 13 weeks — significantly more precise than last menstrual period (LMP) dating, which can be off by 2–3 weeks.

This calculator offers two established formulas: Robinson (1975), which is the most widely used international standard, and Hadlock (1992), an alternative validated in large populations. Both produce results within 1–2 days of each other. When a discrepancy of more than 7 days exists between CRL dating and LMP dating, obstetric guidelines (ACOG, NICE) recommend using the CRL-based date.

Enter your CRL measurement in mm or cm, optionally provide your LMP date for comparison, and receive gestational age, estimated due date, method comparison, and a comprehensive CRL reference table with first-trimester developmental milestones.

When This Page Helps

Accurate gestational dating supports timing prenatal screening, scheduling follow-up scans, and comparing ultrasound dating with LMP dating when the dates differ. CRL is widely used for first-trimester dating because it is more precise than LMP alone.

How to Use the Inputs

1. Enter the crown-rump length measurement from your ultrasound report.
2. Select the unit (mm or cm).
3. Choose the calculation method (Robinson or Hadlock).
4. Optionally enter your last menstrual period date to compare dating methods.
5. Optionally enter the fetal heart rate for a basic assessment.
6. Review gestational age, EDD, and the CRL reference table.

Example Calculation

Tips & Best Practices

• CRL is most accurate between 7 and 13 weeks gestation. Before 7 weeks, the embryo may be too small for reliable measurement.
• If CRL and LMP differ by >7 days, ACOG recommends using CRL for dating — it's more accurate.
• After 14 weeks, CRL becomes less reliable because the fetus begins to curl; biparietal diameter (BPD) is used instead.
• Nuchal translucency screening should be done when CRL is 45–84 mm (roughly 11w0d–13w6d).
• Multiple measurements should be averaged. The longest CRL in a neutral position is most accurate.
• In multiple gestations, each fetus should be measured independently; the largest CRL is used for dating.

The Importance of Accurate Gestational Dating

Accurate dating affects virtually every aspect of prenatal care: timing of first-trimester screening (nuchal translucency, PAPP-A, free β-hCG), scheduling anatomy scan (18–22 weeks), detecting growth abnormalities (IUGR), timing of antenatal corticosteroids for preterm risk, and planning delivery. A dating error of even one week can change clinical management.

CRL Measurement Technique

Proper CRL measurement requires the fetus to be in a neutral position (not flexed or hyperextended), measured in the midsagittal plane. The calipers should be placed at the outer edge of the cephalic pole and the outer edge of the fetal rump. Multiple measurements should be obtained and averaged. Trans-vaginal ultrasound provides superior resolution for early CRL measurements.

Beyond CRL: Later Pregnancy Dating

After 14 weeks, CRL becomes unreliable as the fetus curls. Second-trimester dating uses a composite of biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL). These measurements have progressively wider margins of error as pregnancy advances: ±7 days at 14–16 weeks, ±10 days at 16–22 weeks, and ±2–3 weeks in the third trimester.

Frequently Asked Questions

• What is crown-rump length?

  Crown-rump length is the measurement from the top of the fetal head (crown) to the bottom of the fetal buttocks (rump) on ultrasound. It does not include the limbs. CRL is the most reliable biometric measurement for first-trimester gestational age estimation.

• Which formula is more accurate, Robinson or Hadlock?

  Both are well-validated and produce results within 1–2 days of each other. Robinson (1975) is more widely used internationally and is the standard in UK/NICE guidelines. Hadlock (1992) is commonly used in the US. Either is acceptable for clinical dating.

• When should CRL be used instead of LMP for dating?

  According to ACOG guidelines, if first-trimester CRL dating differs from LMP by more than 7 days, the CRL should be used as the basis for the estimated due date. LMP can be inaccurate due to irregular cycles, uncertain recall, early bleeding, or late ovulation.

• Can CRL detect abnormalities?

  A CRL that is small for dates may indicate growth restriction, incorrect dating, or early pregnancy loss. Abnormally slow CRL growth rate between scans is concerning. CRL alone does not diagnose abnormalities, but it provides the dating framework for nuchal translucency and other screening tests.

• What if my CRL is outside the reference range?

  CRL < 2 mm may be too early to measure reliably — a follow-up scan in 1–2 weeks is recommended. CRL > 84 mm is beyond the optimal dating range, and second-trimester biometry (BPD, HC, FL) should be used instead. Always discuss unexpected measurements with your provider.

• How accurate is CRL dating?

  CRL dating at 7–13 weeks has a margin of error of ±3–5 days (95% CI). This is substantially better than LMP dating (±2–3 weeks) and second-trimester dating (±1–2 weeks). It is the most precise method for establishing gestational age.