The Impact of Dating Twin Pregnancy by the Larger, Smaller or Mean Twin Crown‐Rump Length: A Retrospective Cohort Study

ABSTRACT 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 small for gestational age (SGA) births. Design A retrospective cohort study. Setting A tertiary fetal medicine centre (London, UK). Population or Sample All twin pregnancies between 1998 and 2023 who underwent first trimester CRL ultrasound assessment and fetal growth assessment. 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 SGA < 10th centile and extreme PTB rates (< 28 weeks). Results In the 1129 twin pregnancies, median CRL‐S was 61 mm (interquartile range [IQR]: 56.0–66.0) and CRL‐L was 63 mm (IQR: 58.4–68.9) 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 extreme PTB ( p = > 0.999 and p = 0.765 respectively). Conclusions Dating by the smaller, larger or mean twin CRL does not significantly alter rates of extreme preterm birth, SGA detection or SGA birth. Dating by the mean twin CRL reduces stigmatisation of the smaller twin and retains the utility of accurate gestational age assessment without impacting clinical outcomes.