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182 P. Arbeille et al.<br />

Fig. 13.5. a Evolution of the cerebral (CRI) and<br />

umbilical (URI) vascular resistances in the same<br />

fetus during a normal gestation. The fluctuations<br />

in parallel on the two indices are related<br />

to the variations of the fetal heart rate. Note<br />

that the cerebral resistances remain superior<br />

to the placental resistances at any gestational<br />

age. b Evolution of the C/U ratio during the<br />

same normal gestation (CRI>URI ? normal<br />

CRI/URI always >1.1)<br />

3<br />

Fig. 13.6. Brain-sparing effect. Note the absent diastolic<br />

flow on the umbilical velocity waveform and increased<br />

diastolic flow on the cerebral velocity waveform in relation<br />

to a decrease in the cerebral vascular resistance by vasodilation<br />

(growth-retarded and hypoxic fetus delivered at<br />

35 weeks). S systolic peak, D end-diastolic velocity

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