An ultrasound examination in a woman at 9 weeks' gestation demonstrated a gestational sac with a live fetus in the left uterine horn which was surrounded by an extremely thin outer lining. An interstitial pregnancy was suspected and laparoscopy was performed. During laparoscopy the purple bulge in the left horn gradually reduced in size and eventually disappeared following the external pressure by the tip of the forceps. We believe this pressure caused contraction of the superficial myometrial fibers resulting in a shift in the location of the gestational sac to the central area of the uterus. The pregnancy continued and on Cesarean section at 34 weeks' gestation, a membranous protrusion of the uterine wall in the left horn was noticed. A myometrial defect caused by an intrauterine intervention in the patient's previous pregnancy was suspected to be the cause of this "movable gestational sac" phenomenon.