First-trimester diagnosis of cesarean scar ectopic pregnancy.

Once considered extremely rare, implantation of a pregnancy within the scar of a previous cesarean section is becoming more common. In fact, its incidence is now higher than that of cervical ectopic pregnancies. We identified 5 cases of ectopic pregnancy implanted in a prior cesarean section scar at our institution since 2004. We outline the criteria for the first-trimester sonographic diagnosis of cesarean scar ectopic pregnancy, including a new sign of lower uterine segment ballooning, which has previously not been reported. Clinicians must have a heightened awareness of this serious and potentially fatal pregnancy complication.