First-trimester ultrasound dating of twin pregnancy: are singleton charts reliable?

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.