• Richard Smith
    Department of Maternal & Fetal Medicine, Institute of Reproductive & Developmental Biology, Faculty of Medicine, Imperial College of Science, Technology and Medicine, Hammersmith Campus, Du Cane Road, London
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      I thank Dr Sedgwick et al for writing with their results, which support our findings, and for agreeing that fentanyl should be used with caution in this setting. Measurement of umbilical blood flow, as suggested, may help clarify the mechanism, although in pilot studies in sheep we found difficulties restraining the ewe made obtaining good quality flow velocity waveforms with ultrasound problematic, while anesthetizing the ewe may affect the underlying mechanisms. Implanting flow probes in or around the umbilical cord is technically challenging. I wish Dr Sedgwick luck with future studies.

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