Primary anomalies of villous angiogenesis are rare, and include mainly chorioangioma and molar villi. Secondary anomalies are more common, and are found within the Figure 5.6
Color Doppler mapping and spectral analysis of the umbilical cord blood flow at 7 weeks (a), 11 weeks (b), 14 weeks (c), and 16 weeks (d). Note the progressive appearance of the end-diastolic flow.
(a) (b)
(c) (d)
fetal growth restriction. Histomorphometric features are consistent with previous findings that increasing severity of abnormal Doppler waveforms in the uterine and umbilical circulations is associated with fetal distress and hypoxia. 100
Abnormal intraplacental blood flow at 28 – 34 weeks of gestation is also strongly associated with FGR. 95 As Dop-pler abnormalities of the umbilical circulation are rarely seen in the absence of uterine arterial abnormalities, it is most likely that they are a secondary phenomenon.
Deficient trophoblast invasion during early pregnancy varied, ranging from claims of a reduction in the number
of arteries within the supporting stem villi to a reduction in the capillary vascular bed within the terminal villi, the principal site of gaseous exchange. The underlying cause of the placental lesions is not known, although the fact that Doppler changes in the umbilical circulation are invariably seen subsequent to similar changes in the uterine arteries strongly suggests that they are a secondary phenomenon.
The absence and a reverse of EDF in the umbilical arteries are common findings in pregnancies complicated by severe Figure 5.7
Color Doppler mapping of the umbilical cord placental insertion at 12 weeks (a) and 20 weeks of gestation (b). Note the entry of one of the umbilical arteries inside the placenta (a) and the branching of the chorionic vessels on the surface of the placenta (b).
2D and 3D color Doppler mapping of a free loop of umbilical cord at 16 weeks (c) and 32 weeks (d). Note the difference in coiling of the cord between the second and the third trimester views.
(a) (b)
(c) (d)
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leads to incomplete conversion of the spiral arteries. These vessels remain of higher resistance than normal, and the retention of smooth muscle within the spiral arteries exac-erbates their normal contractility, resulting in longer peri-ods of vasoconstriction and hence greater fluctuations in oxygen tension. This in turn promotes a mild ischemia – reperfusion injury in the placental tissues, 34 leading to oxi-dative stress in the fetal vasculature. Oxioxi-dative stress is a powerful inducer of endothelial cell apoptosis, and repeated insults during mid-pregnancy may lead to regression of the capillaries, particularly as a high percentage are not sta-bilized by a pericyte covering. 100 Such regression would increase vascular impedance in a reverse of the pattern seen during normal pregnancy, and so account for the changes in umbilical waveform observed. The intermedi-ate and terminal villi are the principal sites of gaseous exchange, and decreased vascularization will inevitably impair placental exchange. This will lead to fetal hypoxia and growth retardation, but also reduced oxygen extrac-tion from the intervillous space and so hyperoxia on the venous side of the placenta as a tertiary event. 100
There have been several reports of a fatal outcome of chromosomally normal fetuses between 11 and 18 weeks of gestation following the discovery of absent or reversed EDF in the umbilical artery earlier in pregnancy. 67 , 101 , 102 The umbilical artery PI is also abnormally high in some fetuses investigated with trisomy 18 and triploidy. 102 In these cases, the EDF at 13 weeks of gestation was either incom-plete or absent, suggesting an abnormal development of the villous circulation. These findings indicate that an abnormal EDF in early pregnancy can also be an ominous sign of adverse fetal outcome before mid-gestation as it is during the second half of pregnancy.
Blood flow resistance indices are lower in the superficial and deep placenta compared with the cord insertion area. 49 , 66 Absent or opposite gradient between the umbilical artery and the placental vessels was associated with adverse preg-nancy outcome. 103 Elevated intraplacental resistance to blood flow indicates increased maternal intraplacental impedance as early as week 8 of gestation. 104 Overall, there is no doubt that the Doppler ultrasound study of umbilical artery waveforms helps to identify the compromised fetus in ‘high risk ’ pregnancies, appears to improve a number of obstetric care outcomes, and appears promising in helping to reduce perinatal deaths. 105 Conversely, routine Doppler ultrasound in low risk or unselected populations does not confer benefit on mother or baby. 106
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