Diagnostic ultrasound ( PDFDrive )

1470 PART IV Obstetric and Fetal Sonography
A B C
D
E F
G H
FIG. 43.7 Placental Position. (A)-(D) Transabdominal sonography (TAS) and (E)-(H) transvaginal sonography (TVS) can be used to determine
placental position with respect to the internal os (arrows). If the position is unclear with TAS, TVS should be used. (A) and (E) Complete central
placenta previa. (B) and (F) Complete posterior placenta previa. (C) and (G) Marginal placenta previa. (D) and (H) Low placenta. The calipers show
the distance from the internal cervical os to the leading placental edge.
theory. 57 his theory of “dynamic placentation” suggests that as
the uterus develops, the placenta is “drawn away” from the internal
cervical os. It is unclear whether the primary mechanism is
disproportionate development of the lower uterine segment so
that the placenta, although it does not detach from the uterine
wall, comes to lie more distant from the internal cervical os.
his theory would also be consistent with complete central
placenta previas that do not resolve at a rate approaching that
of other low-lying placentas, because the expansion of the lower
uterine segment would not lead to the resolution of this type of
placenta previa.
If the placenta overlaps the cervix by less than 2 cm at the
end of the second trimester, more than 88% of patients deliver
vaginally. 58 A rate of migration (in the second and third trimesters)
away from the internal os of 3.0 to 5.4 mm per week is also
associated with vaginal delivery, whereas a placental-internal os
distance of less than 2 cm or a pattern of migration of 0.3 to
0.6 mm weekly are associated with interventional cesarean delivery
and a higher rate of peripartum complications. 58,59
he prediction of a placenta previa at delivery is best when
the placenta overlaps the internal cervical os by 1.4 cm at 10 to
16 weeks’ gestation, 60 or 2 cm at 20 to 23 weeks’ gestation. 61
Mustafa et al. 62 demonstrated that if the placenta overlaps by
2.3 cm at 11 to 14 weeks, the probability of a placenta previa at
term is 8%, with a sensitivity of 83% and a speciicity of 86%. 62
Aside from a complete central placenta previa, given the current
data, it is still diicult to predict precisely which patients will
have resolution of their low placenta; therefore further ultrasound
examinations are required to assess placental position if a low
placenta is identiied early in gestation. In a large retrospective
study, if the placenta was low (<2 cm from the internal cervical
os) at 16 to 24 weeks’ gestation, 1.6% had a persistent low placenta
or placenta previa at or near term. 63
For women with a low placenta, the description of the leading
edge of the placenta in the early third trimester as “thin” (≤1 cm
in thickness and/or angle of placental edge <45 degrees) or “thick”
(any other type of placenta) is predictive of delivery complications.
Antepartum hemorrhage is more common with thick-edged
placentas, as is the rate of cesarean delivery, placenta accreta,
low birth weight, and earlier gestational age at delivery. 64 Interestingly,
a more recent study performed in the irst and second
trimesters suggested that a thin-edged placenta with a smaller
angle was more predictive of placenta previa. 65 Although not
ready for clinical implementation, this parameter may help identify
patients who can be reassured early in pregnancy that they will
not have a placenta previa at delivery.
PLACENTA ACCRETA
he normal placenta attaches to but does not invade the myometrium.
At delivery, the placenta separates at the decidual plane,
with an abrupt cessation of intraplacental low as the myometrium
contracts. 66 A placenta that is abnormally adherent to the uterine
wall ater delivery is termed placenta accreta. Placenta increta
occurs if the placenta invades the myometrium more deeply,
and placenta percreta refers to a placenta that at least in part
protrudes through the uterine serosa. Placenta accreta, increta,
and percreta (hereater referred to as “placenta accreta” unless