OP05.07: The clinical utility of a routine 36‐week growth ultrasound in a selected low‐risk population

Determine the sensitivity of a third trimester fetal assessment ultrasound in detecting small for gestational age (SGA) neonates (EFW < 10th centile) and to determine the clinical utility of implementing a routine 36-week fetal assessment in patients who screened low-risk for preterm pre-eclampsia (PE) (FMF UK) in the first trimester. A prospective cohort interventional study involving a subset of pregnant patients enrolled in a larger first trimester screening and PE risk assessment study (IMPRESS study, REB19-0359). Consenting, eligible patients (18- 40 years of age, singleton pregnancy, no maternal comorbidities) who screened low-risk for first trimester preterm PE screening (< 1:100 risk) on the basis of a multifactorial algorithm (FMF UK) were offered a routine 36-week ultrasound (group 1). This was compared to an earlier control group who were offered no additional routine ultrasound follow up after their detailed ultrasound if they had a low risk PE screen except at their care providers discretion (group 2). A local growth chart was used to classify SGA and compared to Hadlock and WHO growth charts. Between Nov 1, 2021, and Oct 31, 2022, 2022 patients met the inclusion criteria and received a routine pregnancy scan at 36 weeks (group 1 = 954, group 2 = 1068). The rate of SGA at birth was comparable to both groups (11.6 vs 12% p = 0.8) The 36-week scan demonstrated 22% sensitivity in detecting SGA at birth in group 1. Sensitivity only modestly improved to 25% if using the WHO or Hadlock growth curves rather than local growth curves. The group 1 though did have a statistically reduced incidence of babies born at < 3rd centile compared to group 2 (4.1 vs 6.9% p = 0.037). Detection rate for breech and oligohydramnios in group 1 at 36 weeks were 6.5 and 0.35%. Routine 36 week growth scan for a selected low risk population demonstrated a poor sensitivity in detection of SGA at birth. However, we did demonstrate a reduced incidence of neonates born < 3rd centile, the SGA group with the highest perinatal morbidity and mortality.