Morphological and ultrasound findings in multiple pregnancy placentation

The incidence of multiple pregnancy has significantly increased over the past decades, reaching different statistics to double, triple, or even overcome these numerical orders globally. Zygosity and chorionicity are the key elements in the multiple pregnancy but the placentation issue should be correlated primarily with zygosity, unlike chorionicity that should be correlated with the outcome and complications of multifetal gestation. Multiple pregnancy is by itself a special maternal-fetal condition, and the monochorionic one, moreover, due to specific complications. These aspects make early assessment of chorionicity and amnionicity a priority. Ultrasound is essential in pregnancy but pathological placental examination after delivery is complementary, in order to have a complete overview of potential mechanisms and pathogenesis affecting twin gestation. In this review, we highlight both ultrasound aspects specific to multifetal placentation, complemented by macro and microscopic morphological aspects, which underpin the obstetric imaging.