Fetal surgery is an evolving field. Some previously fatal diseases can be surgically corrected before birth. Animal models of the procedures have been carefully translated to humans, but less is known about anesthetic techniques. In children, inadequate anesthesia results in greater stress responses as shown by cardiovascular, neuro-endocrine, and metabolic changes. These stress responses have been associated with poorer outcomes. One of the major goals of anesthesia for these procedures is to attenuate this stress response. Fetuses get intramuscular opioids before their incision, and the mother is given high doses of volatile anesthetic (desflurane) with the assumption that whatever amount of desflurane crosses the placenta adequately anesthetizes the fetus. No studies have quantified the fetal levels of desflurane.