الفهرس 
EtiologyOnly 14 pages are availabe for public view
 |  | from | 152 |  |  |
| | | from | 152 | | |
Abstract
Placenta previa is a major cause of obstetrical
hemorrhage, resulting in significant maternal morbidity and
mortality. Although the precise etiology is unknown, many risk
factors including previous cesarean section, advanced maternal
age, and ethnicity have been implicated.
Placenta previa is a condition in which, based on the site
of blastocyst implantation, the maturing placenta will be near to
the internal cervical os. Clinically, although the classic and most
common sign is painless third trimester bleeding, uterine activity
may be evident.
Placentation is a dynamic process, which is thought to
evolve secondary to uterine enlargement. Growth of the uterus
occurs by elongation and hypertrophy of the uterine muscle
fibers, with lengthening of the lower uterine segment and
effacement of the cervix.
These changes result in ‘movement’ of the placenta
away from the internal os, resulting in ‘migration’ of the lowest
margin of the placenta toward the fundus
-113-
Summary
The incidence of placenta previa diagnosed on secondtrimester
ultrasound varies from 5 %to 45%, with one study showing
the incidence decreasing from 32% at 16 weeks of pregnancy to [1,4-
8%] at 24 weeks.
The persistence of placenta previa at the time of delivery has
reported from 0.3% to 8.8%. This supports the hypothesis of
apparent ‘placental migration’.
Ultrasound Color Doppler examination of all cases of major
degree persistent placenta previa will exhibit the characteristic
color Doppler imaging patterns highly specific for placenta previa
accreta.
Massive obstetric hemorrhage is still the leading cause of
pregnancy-related deaths, and placenta previa accreta remains
one of the major predisposing factors. With the increasing rate of
cesarean delivery, the incidence of both placenta previa and
placenta accreta is steadily increasing in frequency. Some
anticipate more cases of placenta previa accreta in obstetric
practice. In several recent series, placenta accreta has emerged as
the major indication for peripartum hysterectomy, accounting for
40-60% of cases. It has, therefore, become a challenging problem
of increasing clinical significance in obstetrics.
-114-
Summary
Several predisposing factors and clinical associations in
placenta previas have been established. These often involve some
type of prior uterine trauma or disturbance to the uterine
vasculature, included the following: increased maternal age,
multiparity, previous cesarean section, previous induced abortion,
previous dilatation and curettage (diagnostic or therapeutic),
multiple pregnancy, and abnormal fetal lie (in the third trimester).
The phenomenon of placental migration is an apparent
change in the position of the placenta with advancing gestational
age, has been well documented since the advent of transabdominal
sonography (TAS). A number of publications have described the
frequent occurrence of a low-lying placenta during the first and
second trimesters, which converts to an upper segment placenta by
the early third trimester It has also been suggested that migration
occurs well into the third trimester.
Transvaginal sonography (TVS) is well established as a safe
and accurate method of placental location that is demonstrably
superior to TAS.
-115-
Summary
It allows accurate visualization of a low-lying placenta at any
stage in pregnancy, and the exact relationship between the edge of
the placenta and the internal cervical os can be accurately measured.
the occurrence of placental migration during the third trimester of
pregnancy using TVS and the rate of migration could be used to
predict the route of delivery.
Follow-up scans on an outpatient basis are indicated when
patients have a diagnosis of placenta previa found incidentally
during an ultrasound examination. Complete placenta previa should
be scanned biweekly. Marginal previas need to be scanned every 3
to 4 weeks. No follow-up scans are necessary to locate the placenta
if it has migrated away from the cervix and the patient is
asymptomatic. If a patient has complete placenta previa at 28 to 32
weeks, she is at risk of bleeding during labor. Hospitalization of
these patients needs to be considered. If bleeding occurs, the patient
needs to be hospitalized.
It is possible that the transvaginal ultrasonographic result
itself influenced management decisions the greater accuracy of
transvaginal uhtrasonography in excluding complete previa
encouraged the physician to manage these cases expectantly.
There is no absolute gold standard for the diagnosis of
-116-
Summary
placenta previa, and where as successful vaginal delivery
excludes clinically significant previa, the performance of a
cesarean section does not prove that vaginal delivery could not
have occurred. Although the outcome measure is not an absolute
one, it does reflect a prevailing clinical standard with which to
compare the ultrasonographic results.
The management of placenta previa has changed
significantly over the last 50 years. Clinical practice has evolved
from an aggressive approach with early delivery to a more
deliberate and expectant form of management with use of
transfusions and close observation in hospital. Prematurity,
aggravated by delivery under adverse conditions such as maternal
hypotension, led to perinatal hypoxia, a greater frequency of
respiratory distress, and fetal anemia, all contributing to a grim
prognosis for the vulnerable immature infant. The overall change
in outcome has been influenced by clinical adjuncts, such as the
availability of blood and component products, the availability of
anesthesia and improved monitoring techniques, amniotic fluid
assessment for fetal lung maturity, tocolytic therapy, and the use
of steroids for the induction of lung maturation.