Determination of gestational age in twin pregnancy: Which fetal crown-rump length should be used?

Aim For twin pregnancy, discrepancies in the crownrump length (CRL) between two fetuses often exist. Evidence is lacking regarding which fetal CRL should be used for estimation of gestational age (GA). Our aim was to determine whether the larger, smaller or the mean CRL is more accurate in determining the GA in the first trimester of pregnancy. Methods This is a retrospective study of twin pregnancies conceived by assisted reproduction. The oocyte retrieval date was used for determination of the true gestational age. CRL dating charts by Robinson, Hadlock and Chitty were used for reference. The values of the larger, smaller and mean CRL were compared with the reference CRL for the corresponding GA, which was obtained from each of the three reference charts. The differences between the reference CRL and measured CRL were calculated. The percentages of which CRL, the larger, smaller or the mean, was closest to the expected reference values were calculated. Results A total of 52 pairs of twins were included in the study. According to Robinson's chart, the proportion of larger, smaller and mean CRL values that were closest to the reference value was found in 11.5%, 75.0% and 5.8% of cases respectively. The larger, smaller and the mean CRLs were closest to the reference CRL in the Hadlock chart for 28.9%, 44.2% and 19.2% of cases, respectively, and closest to the reference CRL in the Chitty chart for 17.3%, 59.6% and 15.4% of cases, respectively. Conclusion The smaller CRL is more accurate in the estimation of the GA for twin pregnancy compared to the larger or mean CRL values.