Diagnostic ultrasound ( PDFDrive )

1466 PART IV Obstetric and Fetal Sonography
Decidual vessel
Maternal
decidua
ET
EVT
Floating
villus
Intervillus space
CT
Fetal vessels
Anchoring
villus
ST
FIG. 43.1 Human Placenta Microarchitecture. Fetal derivatives in the placenta consist of fetal vessels and placental cotyledons (villi). Villi
consist of fetal vessels surrounded by cytotrophoblast cells (CT). Covering the cytotrophoblast cells is a multinucleated cellular layer called the
syncytiotrophoblast (ST). Anchoring villi are in direct contact with the maternal uterine lining, called the decidua. The decidua is traversed by
maternal vasculature. Blood from these vessels empties into the intervillous space and bathes the placental villi. Note that maternal and fetal blood
vessels are separated by trophoblast, villous stroma, and fetal vascular endothelium. Cytotrophoblast cells from anchoring villi can change into an
invasive phenotype called extravillous cytotrophoblast cells (EVT). EVT invade deeply into the maternal decidua. Some EVT, called endovascular
trophoblast cells (ET), embed within the walls of the maternal vasculature. (With permission from Comiskey M, Warner CM, Schust DJ. MHC
molecules of the preimplantation embryo and trophoblast. In: Mor G, editor. Immunology of pregnancy. Austin/New York: Landes Bioscience, 2006.)
become the chorion frondosum and later the placenta. he fetal
side of the placenta consists of the chorionic plate and chorionic
villi. he maternal side consists of the decidua basalis, which
opens up into large cisterns, the intervillous spaces. he fetal
villi are immersed in maternal blood located in the intervillous
spaces. Anchoring villi develop from the chorionic plate. 1 hese
attach to the decidua basalis, holding the placenta in place. 2,3 By
the end of pregnancy, the villi have a surface area of 12 to 14
square meters. 4 his type of placentation, seen in humans and
some rodents, is termed hemochorial placentation.
Placental Appearance
Sonographically, the placenta in the irst and second trimesters
is slightly more echogenic than the surrounding myometrium
(Fig. 43.2A). he attachment site, or base of the placenta, should
be clearly delineated from the underlying myometrium. he edges
of the placenta usually have a small sinus, the marginal sinus
of the placenta (Fig. 43.2B), where intervillous blood drains
into the maternal venous circulation. his area should not be
confused with a placental separation.
Placental lakes (venous lakes, at least 2 × 2 cm) occur in up
to 5% of pregnancies 5-10 (Fig. 43.2C, Video 43.1). hey represent
areas of intervillous spaces devoid of placental villous trees and
are seen as hypoechoic structures within the placenta. Moving
blood low can be seen in these areas. hey may have irregular
shapes or a narrow, cletlike appearance and may change in
appearance over the gestation. Large placental lakes (>5 cm in
largest dimension) have been associated with IUGR. 10
As the placenta matures, areas of echogenicity within the
placenta are visualized (Fig. 43.2D and E). In cases of placental
infarction, there may be hypoechoic lesions with echogenic
borders.
Placental Size
he placenta typically has a round shape with a central umbilical
cord insertion, but variability in the shape of the placenta is
quite common. 11 Placental length is approximately six times its
maximal width at 18 to 20 weeks’ gestation. he mean thickness
of the placenta in millimeters in the irst half of pregnancy closely
approximates the gestational age in weeks. 12 If the placenta
thickness is greater than 4 cm (40 mm) before 24 weeks, an
abnormality should be suspected. hese abnormalities include
ischemic-thrombotic damage, intraplacental hemorrhage,
chorioangioma, and fetal hydrops 13 (Fig. 43.3, Video 43.2).
Given the variable shape of the placenta, calculating a placental
volume from two-dimensional (2D) imaging can be complicated.
Multiplanar volume calculation involves sequential sections of
the placenta at intervals such as 1.0 mm. he margins are manually
traced, and a volume is calculated. 14 Most current studies appraising
the use of three-dimensional (3D) sonography have used
the VOCAL (Virtual Organ Computer-aided AnaLysis) method, 14
in which the 3D volume in question is rotated and the area of
interest traced at its margin, ater which a volume is calculated
(Fig. 43.4).
Placental volume approximation in the irst trimester holds
promise as an important part of early pregnancy evaluation.
Uterine artery Doppler analysis provides limited information
regarding IUGR and the efects of maternal hypertension, but
it is insuicient as a sole indicator of trophoblast invasion, in
part because it is typically performed late in the second trimester.
Small placental volumes in the irst trimester presage abnormal
uterine artery perfusion. 15 Uterine artery Doppler ultrasound
combined with assessment of placental volume may identify
pregnant women at risk for hypertension, abruption, or IUGR. 16,17
First-trimester placental volumes correlate with both placental