Correspondence 411 Fetal blood gases at fetoscopic myelomeningocele repair We read with great interest the case series presented by Baschat et al. 1 , describing serial venous blood-gas analy- ses in three fetuses undergoing fetoscopic myelomeningo- cele (MMC) repair. A baseline sample was obtained by cordocentesis via umbilical venous puncture. The authors state that this was performed prior to CO 2 insufflation of the amniotic cavity and fetal anesthetic medications were administered. After MMC repair was completed, a second fetal umbilical venous sample was obtained. We and others have posed concerns regarding the potential detrimental effect of prolonged CO 2 insufflation on fetal acid–base status 2 . These concerns are related to significant decline in pH reported in the sheep model in similar surgical conditions. Other authors have refuted these findings on the basis of the different placental physiology between the sheep and human models 3 . To our knowledge, no studies on the effect of CO 2 insufflation were attempted in primates before the use of CO 2 became widespread in fetoscopic MMC repair. We would like to pose some questions to the authors of the paper. Why did they elect to administer fetal medication by cordocentesis? Given the length of their reported procedures (149–181 min), this route of administration would be expected to result in a minimal duration of action of the medication. Other reports on fetoscopic repair have used the intramuscular route for fetal analgesia 4 . Might it be that the primary intent was to establish baseline venous gases? The authors go on to perform a second fetal venous sampling at the end of the procedure; however, there does not seem to be any therapeutic rationale for this approach. Was this undertaken simply to provide data to refute the previous concerns regarding use of CO 2 insufflation in humans? The Code of Federal Regulations, issued by the Department of Health and Human Services, has outlined guidelines to local institutional review boards regarding research in pregnant patients and their fetuses (Title 45, Part 46). Section 46.204b states: ‘The risk to the fetus is caused solely by interventions or procedures that hold out the prospect of direct benefit for the woman or the fetus; or, if there is no such prospect of benefit, the risk to the fetus is not greater than minimal and the purpose of the research is the development of important biomedical knowledge which cannot be obtained by any other means.’. Cordocentesis has been associated with a fetal-loss rate of 1% and a risk for fetal bradycardia of 5–10% 5 . Since there was no direct therapeutic benefit to the fetus, how do the authors justify performing a second cordocentesis as being of minimal fetal risk? In conclusion, fetal surgery is a frontier, therefore, there is much to be explored and discovered, but it is also dangerous territory (and in this instance, ethically dubious). Establishing reasonable boundaries requires respecting the precautionary principle. Copyright  2018 ISUOG. Published by John Wiley & Sons Ltd. Ultrasound Obstet Gynecol 2018; 52: 407–412.