Appendix One Crown Rump Length Measurements For Accurate

Appendix One Crown Rump Length Measurements For Accurate Dating Susan Campbell Westerway MApp. Sc 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. 5 wks 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 61 mm - 12 w 3 d, BPD 23 mm 12 w 6 d, HC 80 mm - 12 w 5 d, AC 71 mm – 12 w 5 d, femur 63 mm - 12 wks, humerus 9 mm – 12 wks. Anatomy 13 to 14 Weeks – Spine Curvature and Lie Same fetus. Left image CRL 74 mm = 13 w 3 d. Right image CRL 85 mm = 14 w 2 d. Embryo at 30 days Embryo – GA 8 wks Embryo – GA 10. 5 wks 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. Correct CRL measuring plane. • Statistics used predictive value based on LMP / early scan. At < 11 wks the CRL predictive value is +/- 3 days. This value increased with age to be equal to the BPD, HC & AC (+/- 6 days) from 12 wks 4 days. Gestation < 9 weeks Transvaginal images for head and abdominal measurements were significantly more precise than transabdominal views. (p< 0. 05). At < 13 weeks the CRL had a higher correlation with gestational age. r =. 986, p< 0. 00. 1 No significant difference was seen between the transabdominal and transvaginal crown-rump measurements. 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 17 mm = 8 wks (over measured due to parallax error) CRL 22 mm = 8 w 5 d 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. 11 – 12 week Variations of CRL due to fetal flexion CRL 54 mm = 11 w 4 d CRL 62 mm = 12 w 4 d The most precise results were obtained when the fetus was in the neutral position. Discard any measurement that differs from at least 2 other parameters by more than a week. References 1. Gray’s Anatomy 2. Benson C, Doubilet P. Fetal measurements – normal & abnormal growth. In Diagnostic Ultrasound. Vol 2. Rumack C. ed. 3. Mac. Gregor 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 13 th to 14 th week: a comparison. Scand J Urol Nephrol Suppl. 1987; 104: 165 -7. 227