Getting a better picture: a prospective audit on the accuracy in ultrasound-detection in patients managed as small for gestational age

Placental insufficiency and intrauterine growth restriction are leading causes of stillbirth. To identify the pathologically small fetus for gestational age (SGA), an ultrasound estimated fetal weight (EFW) is referenced against a customised individual growth chart. This audit examined the accuracy of ultrasound-detected SGA compared with actual birth-weight (ABW) and birth-centile, in fetuses managed by delivery in a maternity unit. A prospective audit was conducted for patients with singleton intrauterine pregnancies, who were induced or scheduled an elective caesarean section (ELCS) for the primary indication of SGA, in a maternity tertiary unit from 1st-30th September 2016. Data was collected within a teaching-hospital employing regular trainee competency assessments in ultrasonography. In total, 6% (30/494) of deliveries underwent induction/ELCS for suspected SGA, defined as EFW below the tenth-centile on the customary growth chart. Despite this, three fetuses were on the tenth-centile. The gestational age was equal or greater than 37weeks in 90.3% (range 26weeks + 4day—40weeks + 3 days). The scan to delivery interval averaged at 5.3 days. The birth-centile derived from the ABW detected 56.6% (17/30) with SGA appropriately. 43.4% were managed for SGA, but were not SGA based on birth-centile. Among these (13/30), 46% had an ultrasound difference of >10%, compared with 11.7% in those with birth-centile SGA. Emergency caesarean-section was similar in both groups (38% v 41%). In this audit, those with a higher ultrasound margin of error, were more likely to be above the tenth-birth-centile (Table 5). This study reveals a discrepancy between estimated and birth-centile in a significant proportion of patients. However, in most cases the ABW is within a 10% ultrasound-margin of error. This is similar to other studies. A larger study would better inform on the accuracy of ultrasound-detection rates, establish an acceptable margin of error to improve standards, and inform rates of inappropriate medicalization in misdiagnosed SGA.