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. 2025 Apr;132(5):648-655.
doi: 10.1111/1471-0528.18065. Epub 2025 Jan 9.

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

Gillian V Blayney  1 , Veronica Giorgione  1 , Amar Bhide  1   2 , Basky Thilaganathan  1   2
Affiliations

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

Gillian V Blayney et al. BJOG. 2025 Apr.

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.

Keywords: crown‐rump length; gestational age; perinatal mortality; pregnancy dating; preterm birth; small for gestational age; twins.

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Conflict of interest statement

The authors declare no conflicts of interest.

Figures

FIGURE 1
FIGURE 1
Infographic representation of twin dating by crown‐rump length. Crown‐rump length (CRL) chart illustrating impact of dating by CRL of larger twin (red), smaller twin (blue) and the mean twin CRL (green). Dating by the CRL of the larger twin (x) causes the smaller twin (•) to fall to the lower centiles. CRL centiles obtained from Intergrowth‐21st [https://media.tghn.org/medialibrary/2017/04/GROW_Early_Preg_charts_SIZE_ct_Table.pdf] [assessed 26.02.24].
FIGURE 2
FIGURE 2
Cohort inclusion and exclusion flowchart.
FIGURE 3
FIGURE 3
Twin crown‐rump length measurement distribution. Crown‐rump length (mm) distribution based upon measurement of the smaller twin, mean twin measurement and the larger twin.
See this image and copyright information in PMC

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