Objective: To evaluate the impact of twin dating by ultrasound-measured crown-rump length (CRL) of the larger (CRL-L), smaller (CRL-S) or mean twin measurement (CRL-M) on the rates of preterm birth (PTB) and detection of fetal growth restriction. Design: A cohort study. Setting: A tertiary fetal medicine centre (London, UK). Population or sample: All twin pregnancies between 1998 and 2023 who underwent ultrasound assessment of first trimester CRL and fetal growth. Methods: Data collection included CRL measurement, estimated fetal weight (EFW), pregnancy outcome and birthweight (BW) for each twin. Pregnancies were retrospectively re-dated by CRL-S, CRL-L and CRL-M. Main outcome measures: Small for gestational age (SGA) <10 th centile and PTB rates. Results: Of the 1129 twin pairs median CRL was 61mm (IQR:56.0-66.0) and 63mm (IQR:58.4-68.9) for the smaller and larger twin respectively with a mean discordance of 4.0%. Prenatal SGA diagnosis occurred in 19.8% and 23.1% of smaller twins when dated by CRL-S and CRL-L respectively. When pregnancies were dated by CRL-M versus CRL-S or CRL-L there was no difference in prenatal SGA diagnosis (p=0.275 and p=0.419); SGA at birth (p=0.132 and p=0.325); or PTB (p=1.00 and p=0.765 respectively). Conclusions: Dating by the smaller, larger or mean-twin CRL doesn’t significantly alter rates of extreme preterm birth, SGA detection or SGA birth. Dating by the mean twin CRL reduces stigmatisation of the smaller twin, alleviating parental anxiety, whilst retaining the utility of accurate gestational age assessment without impacting on clinical outcomes.
