Normal Crown-Rump Length at 6 Weeks 5 Days Gestation
At 6 weeks and 5 days of gestation, the normal crown-rump length ranges from approximately 5-9 mm, with an expected mean of approximately 7 mm. 1
Expected Measurements at This Gestational Age
At 6 weeks 5 days (approximately 47-48 days from last menstrual period), the embryonic pole should be visible on transvaginal ultrasound with a CRL typically measuring between 5-9 mm based on normal ranges established in large population studies 1
The embryo becomes visible on transvaginal ultrasound at approximately 6 weeks gestational age, and cardiac activity should be present in an embryo of any measurable crown-rump length at this stage 2, 3
Research data from over 4,600 singleton pregnancies demonstrates that at 6 weeks gestation, the mean CRL is approximately 5-7 mm, with normal variation of several millimeters above or below this range 1
Critical Diagnostic Thresholds
Cardiac activity must be present in any embryo with a CRL ≥7 mm; absence of cardiac activity at this measurement is diagnostic of embryonic demise. 4, 2, 5
For embryos measuring <7 mm in CRL without visible cardiac activity, the finding is concerning but not diagnostic of pregnancy loss, and follow-up ultrasound should be performed in 7-10 days 4, 5
The 7 mm threshold was established to maximize diagnostic certainty and avoid inadvertent harm to viable embryos due to measurement variability 4, 5
Measurement Technique Considerations
CRL should be reported in millimeters (not centimeters) to ensure accuracy, and represents the maximum straight-line measurement from crown to rump, excluding limbs and yolk sac 2
Measurement variability exists between observers, with interobserver limits of agreement of approximately ±14.64% for CRL measurements at 6-9 weeks gestation 6
For a CRL measurement of 6 mm by one observer, the prediction interval for a second observer ranges from 5.4-6.7 mm, highlighting the importance of conservative diagnostic thresholds 6
Common Pitfalls to Avoid
Do not diagnose embryonic demise based solely on absence of cardiac activity if the CRL is <7 mm, as this may represent normal variation in early development or measurement error 4, 5
Avoid using older dating curves (Robinson, Hadlock) for precise measurements at this early gestational age, as they may underestimate gestational age by 3-4 days at 6 weeks compared to more recent data 7
When CRL measurements are close to the 7 mm diagnostic threshold, exercise particular caution and consider repeat measurement or follow-up imaging before making definitive diagnoses 6