Systematic labeling of twin pregnancies on ultrasound

Correct labeling of twin fetuses is needed for consistency in assigning and interpreting longitudinal scan and prenatal screening/diagnostic results. The aim of this study was to describe a standard method of twin labeling in the first trimester of pregnancy and to assess the robustness of such a technique in predicting the presenting twin in subsequent scans and at delivery.

This was a retrospective first-trimester study of all twin pregnancies assessed by ultrasonography at our center between 2000 and 2010. The fetus contained in the gestational sac closer to the maternal cervix was designated as Twin 1 and the relative orientation of the fetuses to each other was then defined as either lateral (left/right) or vertical (top/bottom). In discordant-sex twins, their sex and presenting order on the final scan prior to delivery were documented and compared with the sex and birth order at delivery.

A total of 416 twin pregnancies were seen during the study period. At the 11-14-week scan 90.9% of twins were in lateral orientation while the remainder were oriented vertically. None of the vertically oriented twin pairs but 32 (8.5%) of the laterally oriented twin pairs changed their presenting order between the first and the last ultrasound scan prior to delivery. There were 108 discordant-sex twins scanned in the third trimester, of which the birth order changed in 20.3% that were delivered by Cesarean section and in 5.9% of those delivered vaginally.

The study demonstrates that antenatal labeling of twins according to laterality or vertical orientation is reliable. The technique ensures continuity of biometric assessment from serial scans at each visit, and as such should be adopted as the preferred method of twin labeling. Furthermore, the use of orientation for antenatal labeling of twins rather than assignment of a number based on proximity to the cervix, precludes any misconception regarding which twin will be born first and ensures that parents and pediatricians are aware of the significant likelihood of a peripartum switch.