Dating Scan
First ultrasound showing small embryo with heartbeat
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Evidence-based calculations Used in clinical settings worldwide Regular monitoring recommended
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Why: This calculation helps assess important health parameters for clinical and personal wellness tracking.
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🌱 Early Pregnancy (7 weeks)
First ultrasound showing small embryo with heartbeat
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📅 Dating Scan (9 weeks)
Standard dating ultrasound with clear measurement
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🔬 Pre-NT Scan (11 weeks)
Preparing for nuchal translucency screening
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🔄 Irregular Cycle Dating
CRL dating for patient with 35-day cycles
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✨ End of First Trimester
CRL at 12-13 weeks before transitioning to BPD
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CRL Measurement
LMP Comparison (Optional)
Fetal Heart Rate (Optional)
Current Date
For informational purposes only — not medical advice. Consult a healthcare professional before acting on results.
🏥 Health Facts
— WHO
— CDC
What is Crown-Rump Length (CRL)?
Crown-Rump Length (CRL) is the measurement from the top of the embryo's head (crown) to the bottom of its buttocks (rump). This ultrasound measurement is the gold standard for determining gestational age during the first trimester (7-13 weeks). CRL is highly accurate because embryos grow at a remarkably consistent rate during this period, making it more reliable than last menstrual period (LMP) dating for establishing due dates.
Gold Standard Dating
CRL is the most accurate method for pregnancy dating, with accuracy of ±3-5 days in the first trimester.
Growth Tracking
CRL grows approximately 1mm per day, allowing precise monitoring of early fetal development.
Clinical Standard
Used by healthcare providers worldwide to establish accurate due dates and plan prenatal care.
How is CRL Measured and Used?
Measurement Technique
Optimal Conditions
- • Embryo in neutral position (not flexed or extended)
- • Clear sagittal view of the embryo
- • Measurement from crown to rump, excluding limbs
- • Average of 3 measurements for accuracy
When CRL is Most Accurate
- • Best accuracy: 7-10 weeks (±3 days)
- • Good accuracy: 10-13 weeks (±5 days)
- • After 14 weeks: Use BPD instead
When is CRL Dating Used?
Dating Scan
First trimester ultrasound to establish accurate due date
Irregular Cycles
When LMP dating is unreliable due to irregular periods
Unknown LMP
When the last period date is uncertain or unknown
IVF Pregnancies
To confirm dates match expected gestational age
Growth Monitoring
To assess if embryo is growing appropriately
Pre-NT Screening
Confirm GA before nuchal translucency scan
CRL Calculation Formulas
Robinson & Fleming Formula (Standard)
Where CRL is measured in millimeters. This is the most widely used formula.
Example Calculation
GA = 8.052 × √25 + 23.73
GA = 8.052 × 5 + 23.73
GA = 40.26 + 23.73 = 64 days
= 9 weeks, 1 day
Frequently Asked Questions
Why is CRL more accurate than LMP dating?
CRL is more accurate because embryos grow at a very consistent rate in the first trimester, regardless of the mother's cycle length or ovulation timing. LMP assumes a 28-day cycle with ovulation on day 14, which varies significantly among women.
What if my CRL date differs from my LMP date?
According to ACOG guidelines, if the CRL-based date differs from LMP by more than 5-7 days in the first trimester, the ultrasound date should be used. This is because CRL is more reliable for establishing the true gestational age.
What is a normal CRL growth rate?
CRL grows approximately 1mm per day during the first trimester. A CRL that is significantly smaller than expected may warrant follow-up, while measurements within the 5th-95th percentile are generally considered normal.
When should CRL no longer be used for dating?
After 14 weeks (CRL > 84mm), other measurements like biparietal diameter (BPD) become more accurate. The fetus begins to flex and extend, making CRL less reliable. First trimester dating should ideally be done between 7-13 weeks.
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