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Ultrasound Estimation of Fetal Weight by Various Formulas

Resident: Danielle Hardin, MD

Faculty Advisor: Blake Porter, MD

Introduction

Ultrasounds for fetal weight are routinely done during pregnancy, and are important tools that can help guide management decisions. In the case of macrosomia which is generally defined as an EFW ≥4000 grams, a growth ultrasound performed close to delivery can help clinicians and patients determine the best route of delivery to minimize maternal and neonatal morbidity and mortality. The American College of Obstetrics and Gynecology notes that a cesarean section for fetuses with an EFW ≥4500 grams in a diabetic mother and ≥5000 grams in a non-diabetic mother can decrease the risk of adverse outcomes including shoulder dystocia, hemorrhage, obstetric anal sphincter injuries, and emergent Cesarean section. Given the serious complications that can arise in these infants, it is imperative to have a formula that can accurately estimate fetal weight in a macrosomic fetus. Currently, Hadlock IV formula is used to determine EFW in our patients. This formula utilizes fetal biometry to calculate fetal weight. The purpose of this study is to assess the clinical utility of six ultrasound EFW formulas in a population of pregnancies with a sonographic EFW ≥4000 grams. By comparing these estimated weights to the actual weight of the infant after birth, we can determine which formula is best in macrosomic infants.

Methods

This study is a retrospective chart review of women at OU with a singleton pregnancy, an ultrasound performed at OU Prenatal Diagnostics Center between Jan 2010 - July 2020, and an estimated fetal weight of ≥4000 grams using Hadlock IV formula. The primary outcome EFW is calculated using measurement parameters input into various formulas (Hart, Merz, Shepard, Shinozuka and Porter formulas). The calculated EFW will then be compared to birth weight (BW) to determine accuracy using mean absolute error (AE), relative error (RE), and absolute percentage error (APE).

Results

Out of 424 pregnancies, all ultrasounds were completed within 14 days of delivery (mean 4.63 days ± 3.7 days), and a median birth weight was 4241 g. The Merz method had the lowest mean absolute error overall (AE = 301.62 g), but it did not differ significantly from the Hadlock IV method (AE = 310.5 g). The Merz formula also had the lowest absolute percentage error overall (APE = 8.59%), but it did not differ significantly from the Hadlock IV method (APE = 7.43%). The Hadlock IV method had the lowest mean relative error overall (RE = 2.6).

Conclusions

Hadlock IV should continue to be used clinically even in cases of suspected macrosomia.








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