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360 R. O. Bahado-Singh et al.
with actual volume flow. Oz et al. [29] studied renal
artery RI, PSV, and EDV in 147 patients > 41 weeks;
of these, 21 (14.3%) had oligohydramnios. Adverse
outcome was defined as cesarean for fetal distress, 5-
min Apgar < 7, prolonged NICU stay, or perinatal
death. The renal artery RI was significantly higher in
the oligohydramnios group (0.884 vs 0.860; p 41 weeks was performed
by Selam et al. [21]. They compared the renal
artery RI in 10 oligohydramnios cases (AFI < 5 cm)
with 28 cases with normal fluid. An increase in the
renal artery PI was observed. Differences in study design,
inclusion, and exclusion criteria and different
outcomes of interest account for some of the variability
in the studies conclusions. It is possible that more
than one mechanism accounts for oligohydramnios in
prolonged pregnancies. At this point we are unable to
explain why a different mechanism might predominate
in different patients.
Uterine and Uteroplacental Artery
Doppler
As reported for placental insufficiency in IUGR fetuses,
it is possible that a decrease in uteroplacental
flow could also occur as a consequence of prolonged
pregnancy. Several studies, therefore, have been undertaken
to investigate whether Doppler waveform
analysis of uteroplacental circulations could have a
role in the identification of at-risk fetuses in post-date
pregnancies. Uterine arteries, uterine±arcuate arteries,
and radial spiral arteries have been assessed with
Doppler ultrasonography by investigators in different
studies.
A study on 82 pregnancies of at least 287 days gestation
found normal Doppler findings in uterine arteries
but a significant reduction of time-averaged
mean velocity in the descending aorta was associated
with oligohydramnios, meconium-stained fluid, abnormal
NST, and cesarean delivery for fetal distress
[7].
The prospective study of Malcus et al. [9] involving
102 women with more than 294 completed gestational
days found similar Doppler PIs in the uterine
artery compared with values at term. The study found
that abnormal flow velocity waveforms in the uterine
artery had no significant relationship to fetal asphyxia
defined as the presence of umbilical artery pH
£7.10 and/or an Apgar score < 7 at 1 or 5 min and/or
operative delivery for fetal distress.
Uterine±arcuate arteries were assessed by continuous-wave
Doppler ultrasound in 75 pregnancies of at
least 41 weeks' gestation and systolic±diastolic ratio
(S/D) and RI were calculated by Fischer et al. [8].
There was no significant difference in the mean uterine±arcuate
artery S/D or resistance index between
pregnancies with normal and abnormal perinatal outcome.
In the study by Weiner et al. [12] uterine artery
blood flow velocity waveforms were measured transvaginally
in 142 post-term pregnancies combined
with umbilical artery Doppler and antepartum tests,
namely, NST and estimation of amniotic fluid volume.
The authors concluded that in post-term pregnancies
Doppler velocimetry alone (either the uterine or the
umbilical artery) did not by itself improve the ability
to predict abnormal outcome but may increase the
ability to predict the compromised fetus when combined
with additional antepartum tests.
Zimmerman et al. [14] measured Doppler waveforms
in the uteroplacental arteries in the region of
placental implantation in a total of 153 prolonged
pregnancies. The uteroplacental arteries were localized
within the myometrium close to the placental
bed. These corresponded anatomically to the radial±
spiral arteries and RI values were calculated. The investigators
showed that Doppler resistance indices in
the uteroplacental arteries in the region of placental
implantation did not change significantly with increasing
gestation from 41 to 43 weeks' gestation and
sonographic grading of placenta maturity was also
not related to vascular resistance in the uteroplacental
arteries nor to fetal outcome. Finally, they did not
find any correlation between uteroplacental RIs and
perinatal outcome in post-term pregnancy.
Some data are available for very prolonged pregnancies
beyond 43 weeks' gestation. In a study population
of 44 women proceeding to 43 completed
weeks' gestation, perinatal complications were not associated
with an increased uteroplacental vascular resistance
[15].
Taken together, the results of these studies indicate
that there is no major alteration to maternal uteroplacental
flow in those prolonged pregnancies that are
destined to be complicated by fetal compromise. The
underlying mechanism of fetal complication in postdate
pregnancies therefore appears different from the
uteroplacental insufficiency responsible for IUGR occurring
at earlier gestational ages. Even if placental
histological modifications are present in post-date
pregnancies, they do not appear to result in major
changes in Doppler flow velocity in the uteroplacental
circulation. Indeed, whether or not such changes con-