LAUSR

One of the hallmarks of twin pregnancies is the slower rate of fetal growth compared to singleton pregnancies during the third trimester. The mechanisms underlying this phenomenon and whether it represents pathology or benign physiologic adaptation are currently unclear. One important implication of these questions relates to the type growth charts that should be used by care providers to monitor growth of twin fetuses. If the slower growth represents pathology (i.e., fetal growth restriction (FGR) due to uteroplacental insufficiency), it would be preferable to use a singleton growth chart to identify the small twin fetus that is at risk of perinatal mortality and morbidity. If, however, the relative smallness of twins is the result of benign adaptive mechanisms, likely it is preferable to use a twin-based charts to avoid overdiagnosis of FGR in twin pregnancies. In the current review we addressed this question by describing the differences in fetal growth between twin and singleton pregnancies, reviewing current knowledge regarding the mechanisms responsible for slower fetal growth in twins, summarizing available empirical evidence on the diagnostic accuracy of the two types of charts for FGR in twin pregnancies, and addressing the question of whether uncomplicated dichorionic twins are at an increased risk of fetal death compared with singleton fetuses. We identified a growing body of evidence that the use of twin charts can reduce the proportion of twin fetuses identified with suspected FGR by up to 8-fold, result in a diagnosis of FGR that more strongly associated with adverse perinatal outcomes and hypertensive disorders than a diagnosis of FGR based on a singleton-based chart, and without compromising the detection of twin fetuses at risk of adverse outcome due to uteroplacental insufficiency. We further found that SGA twins are less likely to experience adverse perinatal outcomes or to have evidence of uteroplacental insufficiency than SGA singletons, and that recent data question the longstanding view that uncomplicated dichorionic twins are at an increased risk of fetal death due to placental insufficiency. Overall, it seems that based on existing evidence, the of use twin charts is reasonable and may be preferred over the use of singleton charts when monitoring the growth of twin fetuses. Still, it is important to note that available data have considerable limitations and are primarily derived from observational studies. Therefore, adequately-powered trials are likely needed to confirm the benefit of twin charts before their use is adopted by professional societies.