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a Chapter 23 Doppler Velocimetry in Prolonged Pregnancy 355
cases with one or more abnormal outcome of which
6 had IUGR. A total of 5 (3.8%) compared with 2
(16.7%) of abnormal vs normal groups had abnormal
umbilical artery Doppler measurements. This difference
was not statistically significant. The mean umbilical
artery RI was not significantly different between
the two groups. Doppler had a low sensitivity
for predicting an adverse outcome, as did other antepartum
tests such as NST and amniotic fluid volume.
Anteby et al. [13] studied 79 well-dated pregnancies
³41 weeks. Patients at known risk for adverse
outcome based on medical conditions or with abnormal
antepartum testing based on NST or fluid volume
were excluded. In addition, patients scheduled for cesarean
section or planned induction were also excluded.
The authors therefore considered only lowrisk
patients with prolonged pregnancy. The outcome
measures used were moderate to thick meconium and
moderate to severe persistent intrapartum heart rate
abnormality and finally heart rate abnormalities requiring
operative delivery. The umbilical artery S/D
ratio was statistically elevated in cases with FHR abnormalities
requiring operative delivery. No such difference
was observed when the PIs of the two groups
were compared. An umbilical S/D > 2.5 had 60% sensitivity
and 71% specificity for fetal distress requiring
intervention. The authors suggested that a normal
umbilical Doppler identifies post-term pregnancies at
low risk for intrapartum fetal distress.
Zimmerman et al. [14] studied umbilical artery
Doppler RI in 153 well-dated pregnancies ³41 weeks.
Cases with maternal diseases, prolonged rupture of
membranes, malpresentation, and growth-restricted
fetuses were excluded. Outcomes included asphyxia
defined by Apgar scores, low arterial pH or encephalopathy,
thick meconium, neonatal signs of postmaturity,
abnormal FHR patterns in labor, and nonasphyxial
causes of operative delivery such as arrest
of labor. Pregnancies with normal and abnormal outcomes
had umbilical RI values within the 95% confidence
interval used to define normal. Thirty-eight
pregnancies had asphyxial-related complications,
whereas 30 had non-asphyxial complications. The
umbilical RI for the prediction of asphyxia had a sensitivity
of 37% with a specificity of 75%. For non-asphyxial
complications these values are 7% and 75%,
respectively. Overall, the umbilical Doppler was not a
significant predictor of asphyxial or non-asphyxial
outcome based on inspection of the respective receiver-operating
characteristics curves.
Olofsson et al. [15] performed longitudinal comparison
of umbilical Doppler in 34 women who delivered
after 43 weeks. Doppler values were also compared
with those of 32 controls delivered at < 41
weeks. All pregnancies were well dated with the assistance
of mid-trimester ultrasound. The mean Doppler
velocity and estimated blood volume flow in the umbilical
vein was noted to increase significantly on
longitudinal evaluation of the 34 study cases from 42
weeks to delivery. There was no significant change in
the umbilical artery RI over time. Compared with the
control group, the umbilical artery RI was significantly
reduced, consistent with reduced vascular resistance
and increased blood flow in prolonged gestation.
Of the entire study group, 41.2% experienced
one or more complications, namely oligohydramnios,
meconium staining, fetal distress in labor, or birth asphyxia.
The study findings were novel in that the
authors demonstrated reduced placental resistance
and enhanced blood flow in post-term pregnancies.
Presumably enhanced flow would facilitate further
fetal growth in such patients. These findings are at
odds with the generally accepted view of compromised
placental function with aging. An explanation
might be that in the ªnormalº prolonged gestations,
even though placental senescence limits nutrient
transfer, this bottleneck is overridden by enhanced
placental flow. Indeed, it would explain the observation
that macrosomia is a common feature of prolonged
pregnancies. The study by Olofsson et al. [15]
had findings that deviated significantly from the published
literature cited above. Forty-four study cases
were booked from the first trimester and had dating
confirmed based on 16±19 weeks ultrasound. There
were no cases of perinatal death, thick meconium of
the amniotic fluid, meconium aspiration, or low 5-
min Apgar. There were 16 cases with light meconium,
fetal distress requiring operative delivery or birth
asphyxia based on cord pH values. The umbilical
artery pH was significantly less in this compromised
group than in the 28 patients without complications
(0.73 Ô0.14 vs 0.82 Ô 0.14; p=0.03). Dichotomized
analysis of cases with and without fetal distress
showed significantly lower umbilical artery RI in the
group with distress. The same was found when comparing
groups with and without light meconium
staining. The authors explained these unlikely results
by hypothesizing that vasodilatation developed in response
to mild hypoxia possibly through the release
of vasoactive agents. Contradistinction was drawn to
the situation where severe or prolonged hypoxia results
in increased vascular resistance. A transient increase
in placental perfusion reflecting reduced vascular
resistance reportedly has been demonstrated
with acute hypoxia in healthy lambs [16].
Every effective screening test has varying sensitivity
values depending on the false-positive (1-specificity)
threshold that is chosen. As the false-positive
threshold increases, e.g., 10% compared with 5%, the
sensitivity value will generally also increase. It is
therefore misleading and erroneous to compare the
sensitivities of two screening tests without regard to