EP14.01: Small‐for‐gestational‐age twins with isolated lagging abdominal circumference in twin gestations

Serial growth scans are recommended for twin pregnancies to identify fetal growth restriction (FGR; estimated fetal weight (EFW) or abdominal circumference (AC) < 10th percentile. The clinical significance of isolated lagging AC in the absence of EFW <10th percentile remains unclear. We hypothesise that a diagnosis of FGR based only on AC is associated with a lower rate of SGA infants at birth than using EFW <10th percentile. This was a retrospective cohort study of twin pregnancies from 2010-2019. Biometric parameters were calculated using the Hadlock 4-parameter formula for EFW and Hadlock AC percentile formula (Hadlock 1984). Pregnancies were categorised as FGR if they had either AC or EFW< 10th percentile using the last prenatal growth ultrasound between 20 0/7 and 35 6/7 weeks gestation. Pregnancies with major fetal anomalies or Twin–twin transfusion syndrome were excluded. Our primary outcome was SGA infant at birth (<10th percentile, Fenton growth curve). Secondary outcomes included gestational age at birth and maternal characteristics. 805 twin pregnancies were included in our dataset and 708 (88%) met the inclusion criteria. Most twin pregnancies were dichorionic (n = 525, 74%) compared to monochorionic (n = 155/708, 22%). A total of 442 fetuses met criteria for FGR by ultrasound criteria (31.0%, 442/1416). The rate of SGA was higher among pregnancies with isolated AC < 10th percentile compared to fetuses with overall EFW < 10th (41.5% vs. 22%, p < 0.05). The rate of SGA was similar among pregnancies with isolated AC < 10th percentile compared to fetuses with both AC and EFW < 10th percentile (41.5% vs. 40.0%, p = 0.84). Of note, 36% (87/243) infants were found to be SGA without meeting any criteria for FGR. In twin pregnancies with an ultrasound diagnosis of FGR, we found high rates of false-negative and false-positive diagnosis of SGA. Future studies should incorporate other factors such as umbilical artery Doppler assessment into understanding how to best assess risk for SGA in twin pregnancies.