Abstract: We identified 11 studies for potential inclusion. Of those, we included three studies involving 97 women. Two additional studies are ongoing. Two trials examined in-utero fetal tracheal occlusion with standard (postnatal) care in fetuses with severe diaphragmatic hernia. Whilst the trials utilised fetal interventions that were similar, there were important differences in how access was gained to the fetus and in the timing and mode of delivery. Therefore, we did not combine these trials in meta-analysis and the results are examined in separate comparisons. One trial examined the effect of antenatal corticosteroids versus placebo. Overall, the methodological quality of the trials was variable and no data were available for a number of this review's secondary outcomes. In-utero fetal occlusion by maternal laparotomy versus standard postnatal management (one trial, 24 women) For the primary infant outcome (perinatal mortality), there were no data suitable for inclusion in the analysis. There was no difference between groups in terms of long-term infant survival (risk ratio (RR) 1.06, 95% confidence interval (CI) 0.66 to 1.69). In-utero fetal occlusion by minimally invasive fetoscopy versus standard postnatal management (one trial, 41 women) The primary infant outcome (perinatal mortality) was not reported. Minimally invasive fetoscopy was associated with a small reduction in the mean gestational age at birth (mean difference (MD) -1.80 weeks, 95% CI -3.13 to -0.47), but there was no clear difference in the risk of preterm birth before 37 weeks (RR 1.75, 95% CI 0.78 to 3.92). Long-term infant survival (three to six months) (RR 10.50, 95% CI 1.48 to 74.71) was increased with the intervention when compared with standard management, and there was a corresponding reduction in pulmonary hypertension (RR 0.58, 95% CI 0.36 to 0.93) associated with the intervention. There was no difference between groups in terms of preterm ruptured membranes (< 37 weeks) (RR 1.47, 95% CI 0.56 to 3.88) or maternal infectious morbidity (RR 3.14, 95% CI 0.14 to 72.92), and there were no maternal blood transfusions. Antenatal corticosteroids versus placebo (one trial, 32 women) We also included one trial (involving 32 women) examining the effect of antenatal corticosteroids versus placebo. There was no clear difference in the incidence of perinatal mortality (our primary infant outcome) between the group of women who received antenatal corticosteroids and the placebo control (RR 1.24, 95% CI 0.50 to 3.08). Data (mean only) were reported for two of our secondary outcomes (mechanical ventilation and days of hospital admission) but standard deviations (SDs) were not provided. For the purposes of this review and to permit further analysis we have estimated the SDs based on the reported P values reported in the trial report, although our estimation does assume that the SD is the same in both the intervention and control groups. There were no differences between the antenatal corticosteroid group and the placebo control in terms of days of mechanical ventilation (MD 18.00 days, 95% CI -14.77 to 50.77) or days of hospital admission (MD 17.00 days, 95% CI -13.93 to 47.93) . There is currently insufficient evidence to recommend in-utero intervention for fetuses with CDH as a part of routine clinical practice. We identified three small studies, with only one study adequately reporting on the primary outcome of this review - perinatal mortality, and there were few data pertaining to many of this review's secondary outcomes. WIth regard to the administration of antenatal corticosteroids, there remains a gap in current research, and a large multicentre trial with adequate statistical power should be undertaken to answer this unresolved question. More studies are needed to further examine the effect of in-utero fetal tracheal occlusion on important neonatal outcomes and long-term infant survival and health. Long-term follow-up is of particular importance, and should include morbidity and mortality measures. Further studies should examine the benefits of an in-utero intervention on subgroups with moderate and severe congenital diaphragmatic hernia. Indeed, there are three ongoing studies, being conducted by European, North and South American fetal medicine centres, which will contribute to this gap. Ongoing research and any implementation into clinical practice should include standardisation of the procedure, inclusion criteria and long-term childhood follow-up.

Plain-language summary: We included three randomised controlled studies (involving 97 women). The quality of the studies was variable and a number of this review's important outcomes were not reported in the trials. Two studies compared in-utero fetal tracheal occlusion with standard postnatal repair, but differences between the two studies meant that we were unable to combine the data in our analyses. Neither study reported on perinatal deaths. In single studies, in-utero fetal occlusion was associated with a slightly lower gestational age at birth but no clear difference in the risk of preterm birth before 37 weeks; the occurrence of pulmonary hypertension was reduced. there was no difference between groups in terms of preterm rupture of membranes < 37 weeks or maternal infectious morbidity and there were no maternal blood transfusions. Long-term infant survival was improved with in-utero tracheal occlusion in one study, but not in the other. In the third study, antenatal corticosteroids were compared with placebo and there was no difference in the number of perinatal deaths. Nor was there any difference in terms of the number of days that babies were given mechanical ventilation or the number of days babies spent in hospital. We conclude that the current evidence is too limited by small numbers of pregnancies and the variable methodological quality of the trials to recommend intervention (treatment) in pregnancy for women and their unborn babies with CDH. Further high-quality trials are needed in this area. WIth regard to the administration of antenatal corticosteroids, there remains a gap in current research, and a large, high-quality trial should be undertaken to answer this unresolved question. More studies are needed to further examine the effect of in-utero fetal tracheal occlusion on important neonatal outcomes and long-term infant survival and health. Long-term follow-up is of particular importance, and should include morbidity and mortality measures. Further studies should examine the benefits of an in-utero intervention in relation to the severity of the congenital diaphragmatic hernia (i.e. moderate and severe). Indeed, there are three ongoing studies, being conducted by European, North and South American fetal medicine centres, which will contribute to this gap. Ongoing research and any implementation into clinical practice should include standardisation of the procedure, inclusion criteria and long-term childhood follow-up.