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Uterine Artery: Diastolic notch of the uterine artery (Fig. 5.1) disappears from 24 weeks (Fig. 5.2) and RI drops from 0.84 to 0.56 (Figs. 5.3 and 5.4) [1]. If the notch does not disappear by 24 weeks, most women will develop a hypertensive complication of pregnancy.

Fig. 5.1
figure 1

Flow velocity waveform of a uterine artery in a non-pregnant woman showing diastolic notch and RI of 0.8

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Fig. 5.2
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Diastolic notch in 26 weeks of pregnancy

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Fig. 5.3
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FVW of uterine artery in normal 34 pregnancy weeks with disappearance of diastolic notch and RI of 0.4

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Fig. 5.4
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Right uterine artery VF with normal RI, PI and diastolic flow

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Impaired uterine artery flow velocity is identified by (a) a persistent abnormal index, (b) a persistent notch and (c) significant differences between the indices in two vessels.

The commonly used PI with cut-off value of 1.5 is proved to be the best.

5.1 Umbilical Artery [2]

End diastolic flow is present in all pregnancies by 20 weeks. A mature umbilical artery flow velocity waveform is achieved by 28–30 weeks (Fig. 5.5). Indices mid cord or placental insertion are clinically more reliable. Normal RI ranges from 0.5 to 0.7 and S/D ratio ≤ 3.

Fig. 5.5
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Normal FVW of UA displays forward flow with PI of 1.1

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Absent end diastolic velocity is clearly abnormal.

Decreased diastolic flow (RI > 0.7) indicates early placental insufficiency (Fig. 5.6). When there is reversal of flow, it may be a clinical emergency because most of the foetuses die within 2 weeks.

Fig. 5.6
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Normal FVW in spiral branch of placenta with normal RI of 0.4

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The ratio of intraplacental/UA PI and RI of more than one is associated with increased incidence of IUGR, preeclampsia, foetal distress and neonatal intensive care unit admissions.

5.2 Foetal Descending Thoracic Aorta

Normal flow wave pattern (Fig. 5.7) shows RI = 0.82 + 0.1 and PI = 1.83 + 0.3.

Fig. 5.7
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TA FVW with PI of 2.3 indicates normal blood flow

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There is increase in RI and PI of growth retarded foetuses. High PI is suggestive of foetal academia.

Absent end diastolic flow is suggestive of perinatal complications such as respiratory distress syndrome, necrotising enterocolitis and renal failure.

5.3 Foetal Middle Cerebral Artery [3]

Normal RI of MCA is less than 0.7 and PI is >1.3 (Fig. 5.8). Foetus with mild hypoxia shows reduced umbilical artery flow velocity. The preterminal flow pattern shows absent diastolic flow in the umbilical artery, aorta, vena cava and umbilical vein pulsations. The PI of MCA is significantly lower and the mean systolic velocity is higher in small-for-gestational-age foetuses than the normal foetuses.

Fig. 5.8
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The FVW of MCA displays a continuous forward flow and a high PI of 1.72

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Whether growth retarded foetus is normal, it is determined by the state of umbilical and uterine circulation, and foetal hypoxaemia is determined by MCA Doppler. In growth restriction, a rise in PI of UA precedes changes in the MCA and TA.

5.4 Foetal Venous Circulation [4]

Doppler evaluation of the ductus venosus (Fig. 5.9), hepatic veins and umbilical veins (Fig. 5.9) gives an idea of foetal hypoxic and acidotic state. Absent diastolic velocities and reversal of blood flow in ductus venosus are an absolute indication of delivery.

Fig. 5.9
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FVW of DV displaying continuous forward flow with two surges of velocity peaks and no reverse flow

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The umbilical venous flow when measured at an extra-abdominal level displays regular pulsations up to 15 weeks of gestation. Thereafter, venous pulsations gradually disappear (Fig. 5.10). Occurrence of venous pulsations in UV later in pregnancy is a sign that indicates congestive heart failure in compromised foetuses.

Fig. 5.10
figure 10

Normal UV FVW with its steady flow and no venous pulsations

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