Placenta Previa
Background: Placenta previa literally means afterbirth first, and it defines a condition wherein
the placenta implants over the cervical os. There can be an implantation completely covering the
os (total placenta previa), a placental edge partially covering the os (partial placenta previa), or
the placenta approaching the border of the os (marginal placenta previa). A low-lying placenta
implants a half to a third of the uterus distinct from the os in the caudad.
Etiology: The underlying cause of placenta previa is unknown. There is, however, possible
factors that might result to placenta previa:
This bleeding is thought to occur secondary to the thinning of the lower uterine segment
in preparation for the onset of labor. The placental attachments become disrupted or tear
with this thinning process and cervical dilatation.
When this bleeding occurs at the implantation site in the lower uterus, the uterus is unable
to contract adequately and stop the flow of blood from the open vessels. This is not an
issue with placental implantation in the upper uterus secondary to a larger volume of
myometrial tissue able to contract and constrict bleeding vessels.
Other causes of hemorrhage in the setting of placenta previa include digital examination
and sexual intercourse.
Epidemiology: Placenta previa affects 0.3% to 2% of pregnancies in the third trimester and has
become more evident secondary to the increasing rates of cesarean sections
Mortality/Morbidity: The perinatal mortality rate associated with placenta previa ranges
from 2-3 %.
Age: Age is associated with a varying incidence of placenta previa. The risk of placenta
previa in relation to age is as follows:
Aged 12-19 years - 1%
Aged 20-29 years - 0.33%
Aged 30-39 years - 1%
Older than 40 years - 2%
Pathophysiology: Placenta previa is the complete or partial covering of the cervix. It is initiated
by implantation of the embryo (embryonic plate) in the lower (caudad) uterus. With placental
attachment and growth, the cervical os may become covered by the developing placenta. A
defective decidual vascularization exists, possibly secondary to inflammatory or atrophic
changes. A low-lying placenta is where the edge is within 2 to 3.5 cm from the internal os.
Marginal placenta previa is where the placental edge is within 2cm of the internal os. Nearly
90% of placentas identified as "low lying" will ultimately resolve by the third trimester due to
placental migration. The placenta itself does not move but grows toward the increased blood
supply at the fundus, leaving the distal portion of the placenta at the lower uterine segment with
relatively poor blood supply to regress and atrophy. Migration can also take place by the growing
lower uterine segment thus increasing the distance from the lower margin of the placenta to the
cervix.
Treatment/Management:
Medical Care:
If placenta previa is identified serendipitously (through an ultrasound ordered for some
other reason), continue expectant management until bleeding occurs.
-Once bleeding or contractions occur, the patient must rapidly return to the
hospital for further evaluation.
-The appropriate candidate for home management must be compliant and have
accessible transportation, assistance, and sufficient cognitive function to ensure
comprehension of instructions.
Preterm labor can present as painless vaginal bleeding with placenta previa.
-Magnesium sulfate is the tocolytic of choice. A 6-g loading dose followed by 3
g/h or more is required to reduce uterine irritability.
-Because of the conflicting information regarding beta-mimetics producing
maternal hypotension and tachycardia in the presence of hypovolemia, many
clinicians avoid its use.
-Exercise care to exclude abruption from the differential diagnosis before
tocolysis is undertaken, and continuous fetal monitoring is required during
tocolysis.
Surgical Care:
Cesarean section is the safest mode of delivery for patients with complete placenta previa
or significant hemodynamic compromise.
If time permits, regional anesthesia is the better alternative because general anesthesia is
associated with increased blood loss and the need for blood transfusion.
Most often, the low transverse uterine incision is used; however, a vertical uterine
incision may be used when concern about an anterior placenta and fear of fetal bleeding
exists.
Source:
https://www1.cgmh.org.tw/intr/intr5/c6700/OBGYN/f/web/Placenta%20Previa/
index.htm#:~:text=Pathophysiology%3A%20Placenta%20previa%20is
%20initiated,covered%20by%20the%20developing%20placenta.
https://www1.cgmh.org.tw/intr/intr5/c6700/OBGYN/f/web/Placenta%20Previa/
index.htm#:~:text=Pathophysiology%3A%20Placenta%20previa%20is
%20initiated,covered%20by%20the%20developing%20placenta.
