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Normally, the placenta is attached to the uterus in an
area remote from the cervix. Sometimes, the placenta is located in such a way that it
covers the cervix. This is called a placenta previa.
There are degrees of placenta previa:
A complete placenta previa means the entire cervix is covered. This
positioning makes it impossible for the fetus to pass through the birth canal without
causing maternal hemorrhage. This situation can only be resolved through cesarean section.
A marginal placenta previa means that only the margin or edge of the
placenta is covering the cervix. In this condition, it may be possible to achieve a
vaginal delivery if the maternal bleeding is not too great and the fetal head exerts
enough pressure on the placenta to push it out of the way and tamponade bleeding which may
occur.
 Clinically, these patients present after 20 weeks with
painless vaginal bleeding, usually mild. An old rule of thumb is that the first bleed from
a placenta previa is not very heavy. For this reason, the first bleed is sometimes called
a "sentinel bleed."
Later episodes of bleeding can be very substantial and
very dangerous. Because a pelvic exam may provoke further bleeding it is important to
avoid a vaginal or rectal examination in pregnant women during the second half of their
pregnancy unless you are certain there is no placenta previa.
The location of the placenta is best established by
ultrasound. If ultrasound is not available, one reliable clinical method of ruling out
placenta previa is to check for fetal head engagement just above the pubic symphysis.
Using a thumb and forefinger and pressing into the maternal abdomen, the fetal head can be
palpated. If it is deeply engaged in the pelvis, it is basically impossible for a placenta
previa to be present because there is not enough room in the birth canal for both the
fetal head and a placenta previa. An x-ray of the pelvis (pelvimetry) can likewise rule
out a placenta previa, but only if the fetal head is deeply engaged. Otherwise, an x-ray
will usually not show the location of the placenta.
Patients suspected of having a placenta previa
who are not in a hospital setting need expeditious transport to a definitive care setting where
ultrasound and full obstetrical services are available.
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