Accurate antenatal labeling of twins is paramount, in particular if invasive prenatal diagnosis or therapy is planned. Discordance between antenatal and birth order is particularly relevant in some circumstances, for example if there is discordance of fetal abnormalities. In this issue of the Journal, D'Antonio et al. report interesting data on the rate of order switching between twins within the last 5 weeks of pregnancy based upon ultrasound labeling prior to delivery. These authors have shown in a previous publication that twins may switch at the time of birth. In the present twin cohort, they assessed the relationship between scan findings prior to delivery and final birth order. The analysis was based on STORK data, which is a collaborative regional perinatal database. Based upon weight and sex discordance, they conclude that switching occurs in around one third of cases.
The strength of this paper is that the authors used a large regional twin ultrasound database, with data from nine hospitals over a 10-year period. The main limitation is the inability to adjust for some important confounders, most importantly mode of delivery. In their previous study, the authors showed a marked difference in the chances of perinatal birth order switch according to mode of delivery, with the rate of perinatal switch being significantly higher in those born by Cesarean section (20%) compared with vaginal delivery (6%). The high rate of Cesarean section in twin pregnancies may have contributed to the findings of their current study. Unfortunately, data on mode of delivery were not available for most cases, so this potential bias could not be assessed.
Another limitation is the use of weight discordance as a proxy for labeling. The accuracy of fetal weight prediction using ultrasound is limited, which could have introduced an additional bias. The authors tried to address this by performing a secondary check on the perinatal switch rate using fetal sex discordance, demonstrating that birth weight discordance produced similar estimates of the perinatal switch rate.
Although retrospective in nature, the study findings highlight the potential limitations of antenatal ultrasound in predicting birth order, which is important when planning perinatal management in twin pregnancies. A prospective study adjusting for potential confounding variables would overcome the study limitations and would be better able to confirm or refute this important observation.