Diagnostic ultrasound ( PDFDrive )

CHAPTER 43 Sonographic Evaluation of the Placenta 1469
of 3D placental volume and computer analysis of placental
calciication. 39 How well these indices can be used for the prediction
of preeclampsia requires further investigation. First-trimester
3D power Doppler placental indices are unable to diferentiate
between those destined to have growth restriction as compared
to normally grown fetuses. 40 Recent work in an animal model,
however, demonstrating discrimination of maternal and fetal
blood low in the microvasculature of the placenta may allow
further insights into placental blood low and its efects on
placental function. 41
Amnion-Chorion Separation
he amnion normally “fuses” with the chorion early in the second
trimester. Failure of the amnion and chorion to fuse ater 17
weeks is a rare complication of pregnancy, associated with multiple
abnormalities. Previous amniocentesis is a risk factor for amnionchorion
separation. 42 Associated factors may include IUGR,
preterm delivery, oligohydramnios, placental abruption, and
Down syndrome 43 (Fig. 43.6).
Elastography
A preliminary report has attempted to diferentiate subchorionic
hematoma from placenta previa using elastography. 44 More
rigorous evaluations have shown that the use of both shear wave
and strain elastography of the placenta was diferent between
normal pregnancies and those that developed preeclampsia. 45,46
his exciting novel area of placental research ofers much promise
for the future.
PLACENTA PREVIA
he term placenta previa refers to a placenta that is “previous”
to the fetus in the birth canal. he incidence at delivery is
approximately 0.5% of all pregnancies. 47 Bleeding in the second
and third trimesters is the hallmark of placenta previa. his
bleeding can be life threatening to the mother and fetus. With
antenatal detection, expectant management and cesarean delivery,
FIG. 43.6 Chorioamniotic Separation in Second Trimester. Amnion
(short arrow) is separated from the chorion (long arrow).
both maternal and perinatal mortality rates have decreased over
the past 40 years. 48,49 Accurate diagnosis of placenta previa is
vital to improve the outcome for mother and neonate.
he diferentiation of placental position has historically been
performed by digital assessment of the lower uterine segment
and placenta through the cervix. Using this potentially hazardous
method of evaluation, placental position was classiied as complete
placenta previa, partial placenta previa, incomplete placenta
previa, marginal placenta previa, low-lying placenta, and placenta
distant from the internal cervical os. hese classiications do not
directly apply to the ultrasound examination of placental position
relative to the cervix. he use of ultrasound to evaluate the position
of the placenta in the uterus has both improved knowledge of
the placenta within the uterus and simpliied terminology with
respect to placental position (Fig. 43.7). Complete placenta
previa describes the situation in which the internal cervical os
is totally covered by the placenta. Some diferentiate those
placentas that have a portion of placental substance that extends
over the internal cervical os from those that are centrally placed
over the cervix, a so-called central placenta previa. Marginal
placenta previa denotes placental tissue at the edge of, or
encroaching on, the internal cervical os. A low placenta is one
in which the placental edge is within 2 cm, but not covering any
portion, of the internal cervical os. he terms incomplete placenta
previa and partial placenta previa have no place in the current
sonographic assessment of placental position and should be used
only by a clinician performing a digital examination when a
“double setup” is necessary to determine where the leading edge
of the placenta lies.
Transabdominal scanning can be used to visualize the internal
cervical os and to determine the relation of the placenta to the
cervix in most cases. Factors that can adversely afect the visualization
of the cervix include prior abdominal surgery, obesity, deep
or low position of the fetal head or presenting part, overilled
or underilled maternal bladder, or uterine contractions. Transvaginal
sonography (TVS) is safe 50 and accurate in depicting the
internal cervical os. he proximity of the cervix to the vaginal
probe allows higher-frequency probes to be used, with better
resolution and thus better visualization of the internal cervical
os. With improved resolution, clinicians can accurately determine
the position of the leading placental edge to the internal cervical
os. he use of TVS has been shown to change the assessment of
the placental location in 25% of cases when the placenta is within
2 cm of the internal cervical os, as identiied with transabdominal
sonography. 51 A leading placental edge greater than 2 cm from
the internal cervical os is associated with vaginal delivery, and
distances less than 2 cm are associated with bleeding, potentially
leading to cesarean delivery. 52,53
Although placenta previa can occur in nulliparas, risk factors
include number of prior cesarean deliveries (odds ratio: 4.5 for
one; 44.9 for four 54 ), increasing parity independent of number
of prior cesarean deliveries, 55 and increasing maternal age. 56
Early in the second trimester, the placenta occupies a relatively
large portion of the uterine cavity and oten is positioned near
the cervix. As the uterus grows, a lesser proportion of placentas
are located near the internal cervical os. his relative change in
placental position is best understood by the placental migration