Ultrasound determination of chorionicity in the first trimester has a high accuracy, but it is associated with some pitfalls. This report presents changes in ultrasound findings during a monochorionic pregnancy with chorionic membrane folding (CMF). The patient was a 32-year-old woman, gravida 2 para 0. Her transvaginal ultrasonography identified two gestational sacs (GSs) and two embryos at 7 weeks of gestation. At 9 weeks’ gestation, an ultrasound image showed a lambda sign at both sides and the interruption of chorionic membranes, resulting in the diagnosis of a monochorionic diamniotic (MCDA) twin pregnancy with CMF. At 11 weeks’ gestation, an ultrasound image showed a lambda sign at one portion of the septum and a T sign at another portion. This change suggested that the folded chorionic membrane had partially flattened. At 35 weeks’ gestation, an emergency cesarean section was performed. Two healthy male neonates were delivered. Histological placental examination confirmed that the intertwin membrane was composed of two amniotic membranes without a folded chorionic membrane, confirming the diagnosis of a MCDA twin pregnancy. This case presents two important ultrasound chorionicity findings: a monochorionic pregnancy with CMF can show two GSs and a lambda sign and the CMF can flatten or change during the pregnancy.