Ultrasound is the main prenatal tool to diagnose chorionicity and amnionicity in multiple pregnancies. The importance of this differentiation lies on the fact that monochorionic (MC) pregnancies present with higher morbidity and mortality than dichorionic ones (DC), what can be explained by a sum of factors. Firstly, the placental vascular communications that are present in all MC pregnancies, which unbalance between fetal sides promote complications exclusive for this type of twins, such as twin-twin transfusion syndrome (TTTS), twin anemiañpolycythemia sequence (TAPS) and twin reversed arterial perfusion sequence (TRAP). Secondly, monoamniotic (MA) pregnancies are also associated with other exclusive complications, such as intertwin cord entanglement and conjoined twins. Finally, adverse outcomes common to all multiple gestations are more prevalent in MC ones, probably because of unequal placental sharing, such as selective fetal growth restriction (SFGR), single fetal demise and congenital anomalies. Chorionicity and amnionicity should be ideally determined in the first trimester of pregnancy, when sensitivity approaches 100% by the identification of one or more of the following ultrasonographic criteria: two separate placentas (DC), absence of intertwin membrane (MC/MA), lambda (DC) or T sign (MC/DA) at intertwin membrane placental insertion. Other interamniotic membrane characteristics such thickness greater than 1.5 to 2 mm (DC) or counting the number of layers can be used, since DC membranes have 4 layers and MC only 2. After the first trimester, chorionicity and amnionicity determination becomes less sensible (approaching 90%), because identification of the previous features are impacted by fetal crowding, progressive thinning of membranes and lower prevalence of lambda sign with advancing gestational age. However, identification of discordant fetal sexes after the second trimester arises as a reliable marker of DC. Although these reported findings can be taken as general rules for the determination of chorioamnionicity, it's important to emphasize that in rare situations there are some pitfalls that can lead to incorrect diagnosis, such as oligohydramnios in one cavity misleading to a false MA diagnosis, discordant fetal sex in MC pregnancies due to genital malformation or sex chromosome abnormalities, bipartite MC placenta, false positive or negative lambda sign for various reasons. Concluding, providers must be aware of the importance of early chorioamnionicity determination and how to perform it reliably, since this diagnosis is the key to appropriate management of multiple pregnancies.