The objective of this study is to determine if non-correlation between the ultrasound diagnosis of chorionicity with a postpartum placental histopathological study is a risk factor for presenting adverse perinatal outcomes, assumed fact, but it has not been registered yet in the current literature.MATERIALS AND METHODS: transversal and retrolective study.Twin pregnancies cases treated in the HUS between 2014-2019 were evaluated.Cases with a postpartum placental histopathological study report, concerning monochorionic twin pregnancy, were included.The ultrasound / histopathological diagnostic correlation of chorionicity was evaluated between initial obstetric ultrasound and placental study.Stratified sampling analysis was performed according to the presence or absence of diagnostic correlation and according to the time of the obstetric ultrasound.josha.org