Placenta previa is defined as
a placenta that is near or covering the internal cervical os.
Patients who are at increased risk for placenta previa include
those with advanced maternal age, multiparous women, and those
with a prior history of C-section. The most common symptom of
placenta previa is painless bleeding in the third trimester, but
patients may be asymptomatic. However, this does not mean that
all cases of vaginal bleeding are caused by a placenta previa.
Of the 3 to 5% of all pregnancies that are complicated by third
trimester bleeding, only about 10% are due to a placenta previa.
The major complication, greater in a complete previa, is maternal
hemorrhage at time of delivery. Others are premature delivery,
intrauterine growth retardation, and perinatal death, presumably
secondary to premature detachment of the placenta from the lower
Because trophotropism can cause significant
changes in placental position, a previa is usually not formally
diagnosed before 20 weeks unless symptoms are present. When detected
after 20 weeks in an asymptomatic woman, a follow-up study should
be performed at approximately 32 weeks to determine if the previa
is still present. If a woman is symptomatic, particularly vaginal
bleeding, more frequent ultrasound studies are often necessary.
Placenta previa has been classified into varying degrees to include
low-lying placenta, partial previa, and complete previa. A low-lying
placenta is within 2 cm of the os, a partial previa partially
covers the os, and a complete previa extends completely across
the internal cervical os.
The low-lying placenta and partial previa may migrate and alleviate
the need for C-section. This process, which has been defined as
trophotropism, describes the atrophy of the placenta in the region
of the lower uterine segment with differential growth of other
regions of the placenta toward more vascular-rich sites. Research
has suggested that a placenta previa that is likely to need C-section
is one that overlaps the internal cervical os by 20 mm in the
The following four defined methods exist for imaging of the lower
uterine segment for identification of the location of the placenta
in relationship to the internal os: trans-abdominal imaging, trans-labial
imaging, endovaginal and endorectal imaging.
The most useful and inexpensive study is transvaginal ultrasonography
that provides 100% accuracy in identifying a placenta previa.
An alternative would be transrectal ultrasonography that can be
equivalent to the standard transvaginal approach with less potential
trauma to the pregnancy.
The ultrasound diagnosis of placenta previa is based on the position
of the placenta to the internal cervical os. In the second trimester,
in most cases, the lower uterine segment can be visualized through
a distended urinary bladder. If not, it can frequently be evaluated
with an empty urinary bladder scanning through the amniotic fluid.
In the third trimester, however, both of these techniques may
fail. The fetus is larger and may be persistently positioned in
the lower uterus, and vaginal or endorectal scanning may be needed.
Endovaginal imaging is considered the gold
standard for diagnosis of placenta previa. With trans-abdominal
imaging, false-positive results may occur from an over-distended
bladder, myometrial contractions, and shadowing of the cervix
from the fetal head or other overlying fetal parts.
In a nondilated cervix, three ultrasound-based definitions are
used in relation to the closed internal cervical os: a low-lying
placenta where the placenta approaches but does not cover the
os, a marginal (or partial) previa in which the placenta extends
to the edge but does not cross the os, and a complete previa where
the placenta crosses over and covers the internal os
Vasa Previa and velamentous umbilical
Vasa previa describes vessels of fetal origin that completely
or partially cover the internal cervical os.
Vasa previa may arise from velamentous cord insertion or marginal
cord insertion or from the connecting vessels of an accessory
or bilobed placenta. Vasa previa has also been seen with increased
frequency in association with multiple gestations and low-lying
placentas. These vessels are from the fetal circulation and can
lead to significant fetal blood loss and fetal death at the time
of rupture of membranes or delivery.
Sonographic Findings: An obstetric ultrasound should include visualization
of the umbilical cord insertion into the placenta. Endovaginal
ultrasound should be used in patients with risk factors for vasa
previa, including patients in whom the placental cord insertion
is not identified because of the limitations of transabdominal
imaging that may be associated with maternal size or position
of the maternal bladder. Color Doppler or power Doppler will usually
be needed for confirmation of the diagnosis.
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