How good is ultrasound prediction of fetal birth weight at term?

1. How good is ultrasound prediction of fetal birth weight at term? Enya Ho Ranen Reddy

2. Overview: Audit topic Explanation of standard Method of data collection Data presentation and analysis Case analysis Conclusions Recommendations Suggestions for future study

3. Aim: To evaluate the accuracy of Estimated Fetal Weights (EFW) calculated by ultrasound scans done within 7 days of delivery for term [37/40 GA] singleton pregnancies at National Womens Hospital.

4. Objectives: Analyse USS data from 2010 with an interval of ≤7 days from scan to delivery. Identify if the expected standard of care is being met. Identify and analyse cases that fail to meet the standard. Identify possible sources of random errors and bias. Suggest methods for improvement.

5. Standard: “All singleton term pregnancies having an ultrasound-estimated fetal weight within 7 days of delivery achieve a minimal performance of ±15% percentage variation from the fetal birth weight.”

6. Guideline: D12 policy on “Performance of 3rd Trimester Ultrasound”under the Australasian Society for Ultrasound in Medicine (ASUM). Hadlock C-multiparameter formula [based on AC, HC, BPD, and FL] to give the calculated EFW to within 10-15% of the BW, for weights between 1000g and 4000g. Minimum performance.

7. Evidence: Ultrasound is relatively inaccurate in the assessment of EFW with a mean variation of at least 21% from Birth Weight. No universal formula to calculate EFW. AC is the best measure for prediction of EFW, although inter-operator variability exists, particularly for AC estimation. Most EFW formulae overestimate lower weights and underestimate higher weights.

8. Method: Target population (n=1897) inclusion criteria: ≥37/40 gestation at delivery Delivery at NWH Delivery date in 2010 Singleton pregnancy Sample population (n=120) inclusion criterion : Time interval from ultrasound scan to delivery ≤7 days AGFA sonography database to gather data for sample population, stratifying data by month and chronology.

9. Data analysis:

10. Data interpretation:

11. Total number of data points= 120

12. Total no. of data points= 118 (2 BMI not reported) Data points by BMI: 24 = 39 25-29 = 26 30 = 53

13. Total no. of data points= 120 Data points by BW: 2500 = 7 2501-3999 = 94 4000 = 18

14. Total no. of data points= 120 Data points by time to del: 0 = 2 1 = 14 2 = 22 3 = 16 4 = 15 5 = 18 6 = 16 7 = 17

15. Case analysis: Sonographers BW: 4 cases between 2501-3999g, 1 case 4000g EFW: 1 case ≤2500g, 2 cases between 2501-3999g, 2 cases  4000g GDM/Care under maternal diabetes Limitations of imaging 3 NVDs and 2 LSCSs -> No obvious causal relationship between percentage error and the factors evaluated in the failed cases could be extrapolated.

16. Conclusions: 96% of scans achieved the accuracy standard set by formula limitation. Good performance of NWH sonography department. No obvious causal factors identifiable for large percentage errors. Suggestion of 1 case where LSCS may have been inappropriate (Case 5).

17. Recommendations: 1. Routine documentation of the expected variability in EFW on USS reports. 2. Documentation of single parameter disproportionality and its effect on reported EFW 3. Ensure clinician awareness of % limitation of EFW. 4. Encourage service user awareness of this % limitation. 5. Regular audit of measurement quality.

18. Looking ahead: 1. Use of randomisation for sample population selection. 2. Cross referencing USS reports from Concerto. 3. Frame the current audit to assess different gestations. 4. Frame for assessment of SGA and/or macrosomia. 5. Frame for assessment of patients under MFM. 6. Assess impact of multiple pregnancies in comparison to singleton.

19. References: 1. Ministry of Health. Towards clinical excellence: An introduction to clinical audit, peer review and other clinical practice improvement activities. 2002. 2. Australasian Society for Ultrasound in Medicine guidelines - Policy D12 : 3rd Trimester Ultrasound Screening. July 2008. 3. Dudley NJ. A systematic review of the ultrasound estimation of fetal weight. Ultrasound Obstet Gynecol 2005; 25: 80–89 4. Hoopmann M et al. Performance of 36 different weight estimation formulae in fetuses with macrosomia. Fetal Diagn Ther 2010;27:204–213 5. Blumenfeld YJ et al. Ultrasound estimation of fetal weight in small for gestational age pregnancies. The Journal of Maternal-Fetal and Neonatal Medicine, August 2010; 23(8): 790–793 6. Cohen JM. Influence of ultrasound-to-delivery interval and maternal–fetal characteristics on validity of estimated fetal weight. Ultrasound Obstet Gynecol 2010; 35: 434–441 7. Smith GC, Smith MF, McNay MB, Fleming JE. The relation between fetal abdominal circumference and birthweight: findings in 3512 pregnancies. Br J Obstet Gynaecol. Feb 1997;104(2):186-90. 8. Anderson NG, Jolley IJ, Wells JE. Sonographic estimation of fetal weight: comparison of bias, precision and consistency using 12 different formulae. Ultrasound Obstet Gynecol. Aug 2007;30(2):173-9. 9. Chauhan SP, Hendrix NW, Magann EF, Morrison JC, Kenney SP, Devoe LD. Limitations of clinical and sonographic estimates of birth weight: experience with 1034 parturients. Obstet Gynecol. Jan 1998;91(1):72-7 10. Nuham, G. Estimation of Fetal Weight. Emedicine article online. http://emedicine.medscape.com/article/262865-overview. Apirl 15 2010. 11. Burd et Al. Is sonographic assessment of fetal weight influenced by Formula Selection? J Ultrasound Med. 28:1019-1024. 2009.