Utility of Formulas Using Fetal Thigh Soft Tissue Thickness in Estimating Weight in Gastroschisis.

Objectives: To evaluate the utility of the fetal thigh soft tissue thickness (STT) in calculating the estimated fetal weight (EFW) in fetuses with gastroschisis versus the standard formula of Hadlock et al (Am J Obstet Gynecol 1985; 151:333-337) compared to the actual birth weight (ABW).

Methods: A retrospective study of neonates born with gastroschisis delivered at our institution was performed. Two reviewers measured the fetal thigh STT on saved images. The estimated gestational age, fetal biometric measurements, and ABW were abstracted. In addition to the Hadlock formula, 3 STT-based formulas reported by Scioscia et al (Ultrasound Obstet Gynecol 2008; 31:314-320) and Kalantari et al (Iran J Reprod Med 2013; 11:933-938) were used to calculate the EFW.

Results: Eighty-two patients with gastroschisis qualified for inclusion in our study. The mean STTs ± SD as measured by readers 1 and 2 were 10.9 ± 2.7 and 10.6 ± 2.7 mm, respectively. Seventeen (21%) fetuses were small for gestational age at birth. The Hadlock formula underestimated the EFW relative to the ABW, with an average difference of -97 g (-3.9%) and - 5.1% in terms of growth percentiles. All of the STT-based EFW formulas overestimated the EFW on average by 327 to 701 g (13%-24%) in terms of weight and 26% to 52% in terms of growth percentiles. The Hadlock formula classified 22 as having intrauterine growth restriction (sensitivity, 65%; specificity, 83%, based on the ABW). None of the STT-based formulas classified any fetuses as intrauterine growth restricted.

Conclusions: In a group of patients with gastroschisis, we found that the EFW by the fetal thigh STT calculation overestimated the average fetal weight in all of our cases.