PLACENTA PRAEVIA
Placenta is said to praevia when it is situated wholly or
partially within the lower uterine segment. Placenta praevia is commonly
described in four types depending upon its relationship to the internal
cervical os (fig 12.2). Clinically type I and II placenta praevia are called
minor degree and vaginal delivery is possible, whereas type III and IV are
called major degree in which vaginal delivery is contraindicated for the risk
of haemorrhage, and caesarean section is the safe route of delivery.
INCIDENCE
The lower uterine segment develops
during pregnancy from the expansion of the isthmus, which is represented by an
arbitrary line in the non pregnant uterus. The isthmus elongates to three times
of its original length in first trimester, and the lower segment is only 0.5 cm
at 20 weeks gestation. When fully developed the lower uterine segment measures
5cm. This size is generally reached in the beginning of the third trimester. By
definition placenta is said to be praevia when the leading edge lies within 5
cm of the internal os. Ultrasound carried out early in pregnancy finds more
placentae with leading edge lying within 5 cm of the internal os, but all of
them are not praevia Because of incomplete development of lower segment. Many of them move
laterally with advancing gestation because of expansion of the lower segment. Hence,
an incidence of placenta praevia of as high as 5-28% is quoted from pregnancies
undergoing ultrasound in second trimester. The incidence of placenta praevia
declines with rising gestation and is reported to be 3% between 28-37weeks and
is seen in 0.5% of the patients at the time of delivery. The issue of incidence
of placenta praevia is further complicated by the controversy of age of
viability. The centers using 20 weeks as age of viability and lower limit for
defining placenta praevia have higher incidence than those using 24 weeks as
age of viability.
PATHOPHYSIOLOGY
Haemorrhage in placenta praevia is
a pathological complication of a continuous physiological expansion of the
isthmus underneath the static placenta. In this process the venous sinuses of
the placental bed are torn and maternal blood is lost. As the placenta is
situated right above the internal os most of the haemorrhage passes out
vaginally with very little remaining behind and is bright red in colour. In
contrast to placental abruption the upper uterine segment is unaffected and has
a normal tone. Therefore the uterus is soft and fetal parts are easily
palpable. The haemorrhage is mainly maternal and the fetus generally remains
unaffected.
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