Study design Participants will undergo FETO with standardized preoperative, intraoperative, post-operative care, and delivery. The FETO will be timed between 27+0 to 29+6 weeks gestation for fetuses with an O/E LHR <25% and between 30+0 to 31+6 weeks gestation for fetuses with an O/E LHR between 25% to <30%. The procedure will be performed under local anesthesia with IV sedation or regional spinal/epidural (as done currently for shunts and laser procedures). Prophylactic tocolysis using indomethacin will begin preoperatively. Fetal analgesia and immobilization will consist of fentanyl, atropine and vecuronium. A 10 Fr cannula, 1.3 mm fetoscope within a 3.3 mm sheath (Karl Storz, Tuttlingen, Germany), and a detachable balloon occlusion (BALTACCI-BDPE, Balt, Montmorency, France) system will be used. Participants will receive IV magnesium sulfate for breakthrough labor and may be discharged on oral nifedipine to treat post intervention contractions. Serial ultrasound measurements of lung volume and LHR will begin within 24-48 hours following surgery and continue weekly. Amniotic fluid level and membrane status will also be monitored at weekly intervals. Ultrasonography for fetal growth will be performed every 4 weeks.