OBJECTIVES: The aim of this study was to utilise T2* relaxometry (an indirect method of quantifying tissue oxygenation) to assess the fetal thymus in uncomplicated pregnancies throughout gestation and in a cohort of fetuses that subsequently deliver very preterm. METHODS: A control group of participants with low-risk pregnancies were recruited and retrospectively excluded if they developed any pregnancy related complications after scanning. Participants were recruited who were deemed to be at very high risk of delivery prior to 32 weeks' gestation and retrospectively excluded if they did not deliver prior to this gestation. All participants underwent a fetal MRI scan on a 3 T system incorporating the fetal thorax. T2 and T2* data were aligned and the mean T2* of the thymus tissue determined. RESULTS: Mean thymus T2* decreased with gestation in control fetuses (n = 49). In fetuses who went on to deliver prior to 32 weeks' gestation (n = 15), thymus volume was reduced as was mean T2* (p ≤ 0.001) as compared to controls. This finding persisted in a subgroup analysis of participants with PPROM (p = 0.002), although not in those with intact membranes (p = 0.067). CONCLUSION: These data demonstrates both a likely reduction in perfusion of the thymuses prior to extreme preterm birth, and also the potential for advanced MRI techniques to better interrogate the fetal immune changes prior to preterm birth in vivo.