Transcript Document

Appendix One
Crown Rump Length Measurements
For Accurate Dating
Susan Campbell Westerway MAppSc DMU AMS
North Shore Obstetric & Gynaecologic Ultrasound - North Shore Private Hospital, St Leonards, NSW
University of Sydney – Dept. of Obstetrics & Gynaecology - Northern Clinical School - Royal North Shore Hospital
Background to the Study
12 Week Transvaginal scan
ASUM Policy on early pregnancies states that the earliest measurement of gestational
age taken in pregnancy should usually be accepted as the definitive assessment, with
subsequent examinations reflecting only fetal growth in the intervening period.
The original definition of a CRL was by Mall in 1907 - the crown rump length is the
sitting height, mid brain to the lowest point of breech. For ultrasound the CRL was
defined as the longest length, excluding the limbs & yolk sac. Benson & Doublet
claimed that in the first trimester “the sonographic estimation of gestational age will be
within 0.5wks of the actual age in 95% of cases.”
How often has a patient arrived for an eighteen week morphology scan, based on
ultrasonic dating in the first trimester, to find the dates are actually a week or more
out? The majority of errors appear to occur with CRL measurements taken from 11
weeks onwards.
CRL 61mm - 12w 3d, BPD 23mm 12w 6d, HC 80mm - 12w5d, AC 71mm – 12w 5d, femur 63mm
- 12wks, humerus 9mm – 12wks.
Anatomy
13 to 14 Weeks – Spine Curvature and Lie
Same fetus. Left image CRL 74mm = 13w 3d.
Embryo at 30 days
Right image CRL 85mm = 14w 2d.
Embryo – GA 8wks
Embryo – GA 10.5wks
Illustrations from Gray’s Anatomy
Aim
• To assess the variability in the CRL measurement of a fetus during an ultrasound
examination.
• To determine the stage at which most measuring variables occur – thereby
giving an inaccurate EDD.
• To determine the best combination of fetal parameter measurements for
the most accurate dating.
Methodology
Results
• 215 pregnancies up to 14 weeks gestation by LMP (noted
cycle lengths) / early scans.
• Trans abdominal followed by trans vaginal scan if indicated.
• If dates by CRL > 11 weeks then measure BPD, head
and abdominal circumference, femur and humerus lengths.
• Measure between the fetal poles – cephalic edge to rump
and assess degree of fetal flexion / extension.
• Minimum of 3 CRL measurements required on each fetus.
• Statistics used predictive value based on LMP / early scan.
At < 11wks the CRL predictive value is +/- 3days. This value increased with age to be equal to
the BPD, HC & AC (+/- 6days) from 12 wks 4days.
At < 13weeks the CRL had a higher correlation with gestational age. r = .986, p< 0.00.1
Correct CRL measuring plane.
No significant difference was seen between the transabdominal and transvaginal crown-rump
measurements.
Transvaginal images for head and abdominal measurements were significantly more precise
than transabdominal views.(p< 0.05).
Gestation < 9weeks
Long bones showed the least correlation prior to 13 weeks. r = 0.82, p< 0.01.
Common mistake – yolk sac included in measurement & ill defined end points.
CRL 17mm = 8wks (over measured due to parallax error) CRL 22mm = 8w 5d
Conclusion
This study finds that CRL measuring discrepancies are more prevalent from 12 weeks gestation.
The CRL was the measurement of choice < 12 weeks gestation. From 12 weeks gestation the
CRL should be used in conjunction with BPD, HC and AC.
Long bone measurement accuracy was best from 13 weeks gestation.
The most precise results were obtained when the fetus was in the neutral position.
11 – 12 week Variations of CRL due to fetal flexion
CRL 54mm = 11w4d
Discard any measurement that differs from at least 2 other parameters by more than a week.
CRL 62mm = 12w4d
References
Gray’s Anatomy
Benson C, Doubilet P. Fetal measurements – normal & abnormal growth. In Diagnostic
Ultrasound. Vol 2. Rumack C. ed.
3. MacGregor S, Tamara R, Sabbagha R. et al. Under-estimation of gestational age by
conventional CRL dating curves.Obstet Gynecol 1987:70:344-348.
4. Selbing A. Conceptual dating using ultrasonically measured fetal femur length & abdominal
diameters in early pregnancy. Br J Obstet Gynaecol 1986;93:116.
5. Pedersen J. Fetal CRL or BPD for assessing gestational age in the 13th to 14th week: a
comparison. Scand J Urol Nephrol Suppl. 1987;104:165-7.
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