AETIOLOGY OF PLACENTA PRAEVIA
Placenta praevia is caused by the implantation of the
blastocyst at a site low in the uterine cavity possibly because of rapid embryo
transport from the fallopian tube to the uterine cavity before the endometrium
is receptive, or the implantation site offers special attraction due to
previous endometrial damage. The placenta praevia is seen more commonly in the
following conditions.
i.
Epidemiological factor.
Placenta praevia is three times more common in
women over 35 years compared with those less than 20 years. The incidence also
increases with parity but remains unaffected by the social class.
ii.
Endometrial damage.
Previous history of dilatation and curettage,
spontaneous abortion, and evacuation of retained products of conception, all
have a known association with placenta praevia possibly because of endometrial
damage sustained at that time.
iii.
Uterine scar.
The incidence of placenta praevia increases with the rising
number of previous caesarean sections and is reported to be 0.65% after one and
10% after 4 caesarean sections. A low myomectomy scar is also considered a risk
factor. Failure of expansion of the lower segment because of scar tissue is the
probable cause.
iv.
Uterine pathology.
Placenta praevia is commonly seen in the uteri
affected by endometritis, submucous fibroid, adenomyosis and intracavitary
adhesions.
v.
Smoking.
Smoking during pregnancy increases
the risk of placenta praevia and compensatory placental enlargement due to
carbon-monoxide hypoxaemia has been proposed as a mechanism responsible for
this association.
vi.
Placental problem.
A large placenta in twin pregnancy,
placenta membranacea, succenturiate lobe, bipartite placenta, fenestrated
placenta, and battledore placenta, and velamentous cord insertion are all
associated with placenta praevia.
vii.
Past history.
Patients with previous history of placenta praevia have a
recurrence rate of 4-8%
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