Reduced placental oxygenation during subclinical uterine contractions as assessed by BOLD MRI

OBJECTIVES: During placental Blood Oxygen Level Dependent (BOLD) Magnetic Resonance Imaging (MRI), we have observed spontaneous reductions in placental oxygenation lasting 2-4 min. We hypothesize, that these reductions in placental oxygenation are caused by subclinical uterine contractions.

METHODS: We evaluated placental oxygenation during a five-minute placental BOLD MRI in 56 normal pregnancies (gestational week 23-40) and observed a spontaneous reduction in eight cases. The 56 BOLD MRIs were systematically analyzed for signs of uterine contractions, i.e. visual changes in uterus shape and reductions in the number of pixels within Regions of interest (ROI) covering the outline of the entire uterus.

RESULTS: The eight reductions in the BOLD signal lasted for 217 ± 51 (mean ± SD) seconds with an average signal loss of 17 ± 5%. They were all associated with a contraction, which started 43 ± 21 s prior to the start of the reduction and ended 71 ± 30 s prior to the end of the reduction. In the remaining 48 MRIs, we observed no contraction.

CONCLUSION: We suggest that the observed spontaneous reductions in placental oxygenation are caused by uterine contractions. According to our data, subclinical uterine contractions occur regularly and have a markedly impact on placental oxygenation. Therefore, uterine contractions need to be considered in the interpretation of placental MRI as they may interfere with the MRI results.