First-trimester ultrasound dating of twin pregnancy: are singleton charts reliable?
Article first published online: 19 MAY 2010
© 2010 The Authors Journal compilation © RCOG 2010 BJOG An International Journal of Obstetrics and Gynaecology
BJOG: An International Journal of Obstetrics & Gynaecology
Volume 117, Issue 8, pages 979–984, July 2010
How to Cite
Dias, T., Mahsud-Dornan, S., Thilaganathan, B., Papageorghiou, A. and Bhide, A. (2010), First-trimester ultrasound dating of twin pregnancy: are singleton charts reliable?. BJOG: An International Journal of Obstetrics & Gynaecology, 117: 979–984. doi: 10.1111/j.1471-0528.2010.02592.x
- Issue published online: 8 JUN 2010
- Article first published online: 19 MAY 2010
- Accepted 7 April 2010. Published Online 19 May 2010.
- Crown–rump length;
- pregnancy dating;
- singleton CRL charts;
- twin pregnancy dating
Please cite this paper as: Dias T, Mahsud-Dornan S, Thilaganathan B, Papageorghiou A, Bhide A. First-trimester ultrasound dating of twin pregnancy: are singleton charts reliable? BJOG 2010;117:979–984.
Objective The aim of this study was to assess the performance of validated singleton crown–rump length (CRL) formulae in dating twin pregnancies at 11–14 weeks of gestation.
Design Retrospective cohort study.
Setting Fetal medicine unit of a London teaching hospital.
Sample Three hundred and eighty-four pregnancies with known dates of conception.
Methods Retrospective analysis of 266 singletons and 118 twin pregnancies conceived by in vitro fertilisation (IVF), with a known date of conception. The gestation calculated from the date of conception was compared with the expected gestation from fetal size using a number of different CRL formulae.
Main outcome measures Difference in gestational age computed from fetal size (CRL) of the bigger and smaller fetus in twin pregnancies and singleton pregnancies using three formulae.
Results Two of the three studied CRL formulae systematically underestimated the mean gestational age and size of singleton IVF pregnancies (Robinson formula: gestation = 1.4 days, size = 2.7 mm). Twin CRL measurements straddled those of singletons, regardless of the CRL formula used (Robinson formula: larger twin gestation = 2.4 days, size = 4.7 mm; smaller twin gestation = 0.8 days, size = 1.7 mm). These underestimates in gestation and size for IVF singleton and twin pregnancies are well within the known limits of accuracy of first = trimester ultrasound measurements, and are of limited clinical significance.
Conclusions Currently available CRL charts underestimate both the age and size of IVF singleton pregnancies by a clinically insignificant amount. This difference is similar for twin pregnancies, suggesting that singleton CRL charts can be used to date twin pregnancies accurately.