Normal Crown-Rump Length at 8 Weeks and 5 Days
At 8 weeks and 5 days of gestation (61 days), the expected crown-rump length is approximately 16-18 mm based on international standards.
Expected CRL Range
Using the INTERGROWTH-21st international standards equation (mean CRL = -50.6562 + (0.815118 × GA) + (0.00535302 × GA²), where GA = 61 days), the expected mean CRL is approximately 17.3 mm 1
The standard deviation at this gestational age allows for normal variation of approximately ±2-3 mm around the mean value 1
This measurement corresponds to the optimal window for first-trimester dating, where CRL provides accuracy within ±5-7 days when measured between 8-13 weeks gestation 2
Clinical Context and Measurement Standards
The CRL should be measured as the maximum straight-line dimension from crown to rump, excluding limbs and yolk sac, and must be reported in millimeters (not centimeters) for accuracy 2
At this gestational age, cardiac activity must be visible on transvaginal ultrasound if the embryo is viable, as any embryo ≥7 mm CRL without cardiac activity definitively confirms embryonic demise 2, 3
The American College of Obstetricians and Gynecologists recommends that when discrepancies exist between last menstrual period dating and CRL-based dating, the gestational age determined by CRL measurement should be adopted 2
Important Measurement Considerations
At 8 weeks 5 days, the embryo has transitioned from a C-shaped structure to a more recognizable form, allowing for accurate crown-rump measurement along the longest axis 4
Interobserver variability for CRL measurements at this gestational age is relatively low, with limits of agreement of approximately ±14.64%, meaning measurements should be reproducible within 2-3 mm 5
The measurement becomes visible on transvaginal ultrasound starting at approximately 6 weeks when the embryonic pole first appears, and by 8 weeks 5 days should be clearly measurable 2