CRL to Gestational Age & Due Date
Enter the ultrasound crown-rump length (CRL) to estimate gestational age. Add the scan date to estimate an expected due date (EDD).
Educational use only. This tool does not replace clinical assessment, ultrasound interpretation, or advice from your OB/GYN or midwife.
What is crown-rump length (CRL)?
Crown-rump length is the measured distance from the top of the embryo or fetus (the crown) to the bottom of the torso (the rump). In early pregnancy, this is one of the most reliable ultrasound measurements for estimating gestational age.
Because early fetal growth is usually consistent, CRL is commonly used in the first trimester to date pregnancy and estimate a due date, especially when cycle length is irregular or when the last menstrual period is uncertain.
How this calculator works
This calculator uses a standard first-trimester CRL dating equation to estimate gestational age in days, then converts that to weeks and days. If you provide a scan date, it also estimates your expected due date by counting forward to 280 days (40 weeks) of gestation.
- Input: CRL in millimeters
- Output: Estimated gestational age (weeks + days)
- Optional output: Estimated due date (EDD), if scan date is entered
- Optional comparison: CRL dating compared with LMP dating
Quick CRL reference table (approximate)
| CRL (mm) | Approximate Gestational Age |
|---|---|
| 5 mm | ~6 weeks 1 day |
| 10 mm | ~7 weeks 0 days |
| 20 mm | ~8 weeks 5 days |
| 30 mm | ~10 weeks 0 days |
| 45 mm | ~11 weeks 0 days |
| 60 mm | ~12 weeks 1 day |
| 75 mm | ~13 weeks 2 days |
| 84 mm | ~13 weeks 6 days |
When CRL dating is most accurate
CRL is generally most accurate in the first trimester, especially between about 7 and 13+6 weeks. During this period, differences in individual fetal growth are relatively small compared to later pregnancy.
Why clinicians prefer first-trimester ultrasound dating
- Lower biological growth variation early in pregnancy
- Less impact from maternal/fetal factors seen later in gestation
- Useful when LMP is uncertain, unknown, or cycle length is irregular
Important limitations
- This tool gives an estimate, not a diagnosis.
- Ultrasound measurement quality and fetal position can affect CRL.
- Different clinics may use slightly different dating standards.
- In IVF pregnancies, dating is often based on embryo transfer timing.
- If there is a meaningful difference between LMP and ultrasound dating, your clinician will decide which is most appropriate.
Frequently asked questions
Is CRL better than LMP for dating?
In early pregnancy, CRL from a quality ultrasound is often more accurate than LMP-based dating, particularly when cycle timing is uncertain.
Can this calculator predict my exact delivery day?
No. The due date is an estimate. Most babies are born before or after the exact EDD.
What if my value seems outside normal range?
Always discuss results with your prenatal care provider. They can combine ultrasound findings, clinical context, and follow-up scans for the safest interpretation.
Bottom line
A crown-rump length calculator is a practical way to estimate gestational age in early pregnancy. It is best used as a supportive educational tool alongside professional prenatal care and formal ultrasound reporting.