PLACENTA
PREVIA
Placenta
previa refers to implantation of the placenta in the lower segment of the
uterus, so that it partially or totally covers the internal os of the cervix.
Placenta previa and abruptio placentae account for more than 85% of the cases
of hemorrhage during the last trimester of pregnancy. This is associated with
potentially catastrophic maternal bleeding and obstruction of the uterine
outlet.
Placenta previa is classified into four types, according to the degree with which the placenta superimposes or encroaches on the internal os: total or central placenta previa, in which the internal os is covered entirely; partial placenta previa, in which the placenta partially caps the internal os (10% to 90%); marginal placenta previa, in which only a small edge of the placenta is at the internal os; and low-lying, in which the placenta is located in the lower uterine segment but does not touch the cervical os. These degrees may vary with cervical dilation or gestational age.
Ultrasonographic studies
indicate that the location of the placenta is subject to some degree of
“migration” during the course of gestation. In addition, as the lower uterine
segment elongates late in pregnancy, these relationships may change. Therefore,
the above classification of placenta previa is only relative, and it should be
remembered that if a diagnosis is made of a particular type, it refers to the
time of examination. In the partial and total varieties of placenta previa, a
slight degree of separation of the placenta is inevitable when the lower
segment of the uterus distends, and hence a certain degree of bleeding is bound
to occur.
The incidence of placenta
previa varies from 1 in 100 to 1 in 200 deliveries. The condition is much more
frequent in multiparas than in primiparas, in older patients (older than 35:
1%; older than 40: 2%), with a prior cesarean delivery (two to fivefold
increase), in smokers (twofold increase), following in vitro fertilization, and
in multiple gestations.
Little is known regarding
the etiology of the condition. It has been suggested that defective
vascularization of the decidua, as the result of inflammatory or atrophic
processes, may be a contributing factor for placenta previa. Under these
circumstances, the placenta is forced to spread over a wide area in order to
obtain sufficient blood supply. It is also possible that a multiplicity of
factors contributes to lower implantation of the ovum with extension of the
placenta toward the internal os.
The symptoms of placenta
previa include painless hemorrhage (70% of cases), which usually appears after
the seventh month of gestation. The hemorrhage may come at any time, without
warning and even when the patient is asleep. It usually begins as a slight
intermit- tent bleeding, but it may become profuse without any notice. The mechanisms of bleeding in
placenta previa are poorly understood. Separation of small areas and tears in
the vessels may occur as the consequence of stretching of the uterine walls,
especially the distended lower segment. The blood is maternal in origin.
The diagnosis is usually
not difficult when the classic symptoms are present. Ultrasonography has
replaced other imaging techniques and the classic “double set-up” (vaginal
examination in the operating room so that an emergency operative delivery could
be accomplished should hemorrhage be
precipitated). It is important to remember that any vaginal manipulation may
precipitate extensive hemorrhage.
Because of the overstretched lower segment and abnormalities of placental attachment, profuse bleeding may occur even after the delivery of the fetus. The lower segment may be unable to contract sufficiently to check the bleeding. Placenta accreta occurs in 15% to 25% of cases of placenta previa, particularly in the presence of a previous cesarean section scar.
