Fetal aortic valvuloplasty (FV) aims to prevent fetal aortic valve stenosis progressing into hypoplastic left heart syndrome (HLHS) resulting in a postnatal univentricular (UV) circulation. Despite increasing numbers of FVs performed worldwide, the natural history of the disease in fetal life remains poorly defined. The primary aim of this study was to describe the natural history of fetal aortic stenosis and, secondarily, to test previously published criteria designed to identify cases of evolving HLHS with the potential for a biventricular (BV) outcome following FV. We report the natural history of 107 fetuses in continuing pregnancies that did not undergo FV from a retrospective multicenter study in Europe of 214 fetuses with aortic stenosis (2005–2012). We examined longitudinal changes in Z-scores of aortic and mitral valve and left ventricular dimensions, and documented direction of flow across foramen ovale and aortic arch, and mitral valve inflow pattern and any changes to determine those fetuses satisfying the Boston criteria for emerging HLHS and to estimate the proportion of these that would also have been considered ideal FV candidates. We applied the threshold score where a score of 1 was awarded to fetuses for each Z-score meeting the following: left ventricular length and width > 0; mitral valve width > −2 and aortic valve width > −3.5 and also where the pressure gradient across either the mitral or aortic valve was > 20 mmHg and compared the predicted circulation with known survival and final postnatal circulation (BV; UV or conversion from BV to UV). In the 107 ongoing pregnancies there were 8 spontaneous fetal deaths, resulting in 99 live-born children. 5 were lost during follow up, 5 had comfort care and 4 had mild aortic stenosis not requiring intervention. There was an intention to treat in the remaining 85 but 5 of them died before surgery before the circulation could be determined. Thus 80 underwent postnatal procedures with 44 BV, 29 UV and 7 BV-UV outcomes. 70/85 children (82%) with an intention to treat had ≥30 day survival. Survival was superior in BV circulation at median 6 years (p=0.041). Aortic valve size was significantly smaller at presentation in fetuses with UV outcomes (p=0.004) but its growth velocity was similar in both circulatory outcomes. In contrast the mitral valve (p=0.008) and left ventricular inlet length (p=0.0042) and width (p= 0.0017) were significantly reduced by term in fetuses with UV compared to BV outcomes. Fetal data from 70 treated neonates, recorded before 30 completed gestational weeks was evaluated for emerging HLHS. Forty-four had moderate or severe left ventricular depression and 38 of these had retrograde flow in the aortic arch with a further two having left to right flow at atrial level and a–wave reversal in the pulmonary veins. Thus 40 of the 70 satisfied the criteria associated with emerging HLHS and a BV circulation was documented in 13, (33%). Twelve of the 40 fetuses (30%) had a threshold score of 4 or 5, with a BV circulation in 5 (42%) of them without fetal intervention. Our natural history cohort of children diagnosed with aortic stenosis with known outcomes shows that a substantial proportion of fetuses meeting the criteria for emerging HLHS, with or without favorable selection criteria for FV, had a sustained BV circulation without fetal intervention. This indicates that further work is needed to refine selection criteria to offer appropriate therapy to fetuses with aortic stenosis.