EP14.22: Twin pregnancy outcomes when dating by the larger, smaller or mean Crown–rump length

Expert consensus favours the use of larger twin CRL to date a twin pregnancy, due to concerns over missing fetal growth restriction (FGR). However, this stigmatises the smaller twin, which is displaced to the lower growth centiles, heightening parental anxiety and increasing surveillance. Conversely, there is little data on dating by the smaller twin. This study aimed to evaluate the impact of various first trimester twin dating strategies on the rates of preterm birth (PTB) and FGR detection. In this cohort study included all twin pregnancies (1998-2023) seen in a tertiary fetal medicine centre with available CRL and pregnancy outcome data. Retrospective (re-)dating by smaller twin CRL (CRL-S), larger twin CRL (CRL-L) and mean twin CRL (CRL-M) and the impact on small for gestational (SGA) rate and gestation at birth were evaluated. Of the 1129 sets of twins, median CRL was 61mm and 63mm for the smaller and larger twin respectively with mean CRL discordance of 4.0%. 1.5% of twins had a discordance >15%. When dated by CRL-S and CRL-L, prenatal SGA occurred in 19.8% and 23.1% of smaller twins respectively; whilst equivalent SGA birth rate in the smaller twin was 43.6% and 49.5%, respectively. When comparing dating by CRL-M vs CRL-S, there was no difference in gestational age at birth (p = 0.179), prenatal SGA diagnosis (p = 0.275), SGA at birth (p = 0.132), or preterm birth rate (p = 1.00). There was also no difference in gestational age at birth (p = 0.173), prenatal SGA diagnosis (p = 0.419), SGA at birth (p = 0.325), or preterm birth rate (p = 0.765) when comparing CRL-M vs CRL-L. Dating by the smaller, larger or mean twin Crown–rump length does not significantly alter the prediction of gestational age, rates of extreme preterm birth or the detection of SGA prenatally and postnatally. Given these findings, the policy of dating by the mean twin CRL would minimise parental anxiety in early pregnancy yet retain the utility of accurate gestational age assessment without impacting on the clinical outcomes of twin pregnancies.