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Placenta praevia

What is placenta previa?

Placenta previa refers to implantation of the placenta near the internal cervical os. There are different types of placenta previa: partial placenta previa, complete or total placenta previa, marginal placenta previa and low insertion placenta, which are explained below.

This is one of the most common obstetric complications, and it can cause postpartum hemorrhage during labor.

What is the incidence of placenta previa?

It occurs approximately in 1 of 200 births, but only 20% is total previous placenta. The incidence of placenta previa is 1 in 200 to 1 in 390 pregnant women with gestational age greater than 20 weeks, the frequency of occurrence with deliveries increases, thus for the nulliparous women, the incidence is 0.2%, while in multiparous, it can be greater than 5% and the recurrence rate is 4% to 8%.

What can cause placenta previa?

The causes of abnormal implantation of the placenta are not really known, but hypotheses are established according to uterine and placental causes.

Uterine causes

These alter the endometrium or myometrium causing an abnormal implantation of the placenta, such as:

  • Antecedents of caesarean section
  • Uterine curettage
  • Multiparity
  • Age: over 35 years
  • Short intergenic interval
  • Uterine myomas (myomectomies are four times higher risk)
  • Endometritis
  • History of placenta previa (12 times greater probability of presenting a new episode).

The uterine causes that generate the low implantation of the placenta, as for example the antecedent of cesarean section, indicates that the placenta during the gestation process migrates to places of higher arterial perfusion, but when there are scars, it does not allow the migration of the placenta because of the low levels of irrigation that exist in it.

Placental Causes

They are those that favor the increase in the size of the placental cake or the implantation surface. A reduction in uterine-placental oxygen promotes the increase in its surface generating an obstruction of the internal cervical os. These placental causes are:

  • The twin pregnancy
  • Erythroblastosis
  • Male fetus
  • Smoking and cocaine.

What is the Pathophysiology in placenta previa?

It is normal for the placenta to move as the uterus stretches and grows. At the beginning of pregnancy, it is normal for the placenta to be inserted into the lower part of the uterus (or known to patients as the womb). But as the pregnancy progresses, the placenta moves to the top. During the third trimester, the placenta should be close to the fundus, so that the internal cervical os is clear for delivery.

The lower segment is an inadequate region for placental insertion, because it has a thinner endometrium, the decidua is much thinner, altering its vasculature, leading to a more flat and irregular conformation of the placenta, forming partitions between cotyledons, which is associates that the placenta already developed invades the myometrium, the latter is known as placental accreta; The lower segment of the uterus does not contain the same amount of muscle fibers and collagen fibers predominate, there is greater distension and less capacity to collapse the capillaries, generating alterations in the normoinsertion and functioning of the cord, which together trigger lesions in both the fetus and in the mother.

How is placenta previa classified?

There are 4 types of placenta previa that differ depending on the relationship they have with respect to the hole in the cervix.

  1. Complete / Total:
    1. Complete or total placenta previa is characterized by covering the entire internal cervical orifice of the patient
  2. Partial:
    1. This is the partial placenta previa characterized by covering some percentage of the internal orifice of the cervix.
  3. Marginal:
    1. The location of the placenta is less than 2.5 cm from the edge of the OCI without covering the hole. It is important to bear in mind that marginal placenta previa can generate complications during labor, because the cervix in this period is in the process of dilation and the placenta can obstruct the internal orifice of the cervix, preventing the passage of the fetus to through the birth canal or causing postpartum hemorrhage.

Low Lying Previa This is characterized by being located approximately 3cm from the edge of the OCI. Although in many texts it is not considered within the classification of placenta previa, it can cause the same complications as marginal placenta previa in childbirth, so it should not be forgotten about its existence.

Tipos de placenta previa
Classification of placenta previa

It is important to note that most previous placentas (90%) diagnosed in the middle of pregnancy can be reclassified as normal due to segment formation and uterine growth. Phenomenon called placental migration.

What are the signs and symptoms of placenta previa?

This pathology is part of the group of diseases that make up the hemorrhages of the Third Trimester of Pregnancy. Within the clinic of pregnancy with placenta previa the following symptoms can be highlighted:

Non-painful bleeding

This is a vaginal bleeding that occurs in the third trimester of pregnancy, which is characterized by not producing pain, intermittent and bright red.

  • It is the most important symptom and sign
  • It is precipitated by the contractions of Braxton-Hicks.
  • It can be presented before week 30 (30% of cases), between week 30 and 35 (30%), before labor (30%) and during delivery (10%) without previous episodes of bleeding . Up to 10% of cases occur concomitantly with placental abruption (Abruptio of placenta)
  • The normal course of this first bleeding usually diminishes and disappears, giving time to the correct diagnosis of the present disease.
  • Subsequent bleeding is considered much more aggressive and of vital commitment.

Related to the weeks of gestation has been found relationship; that the bleeding that occurs before week 28 corresponds to a total occlusion of the placenta and a third of the bleeding occurs in labor. Clinically, the bleeding being 35%, is associated with transverse fetal situations, podal presentation, very high cephalic presentation and soft uterus. A placental murmur can be heard in the hypogastrium.

There is also the maternal risk of secondary hypovolemia that can lead to death due to multiple organ involvement and / or secondary infection.

Regarding fetal compromise, there is an increased risk of intrauterine growth restriction (IUGR) up to 16% due to poor circulation of the uterine segment.

How is placenta previa diagnosed?

The diagnosis of placenta previa can be made by the clinical part based on the patient’s history and pain-free transvaginal bleeding during the second half of pregnancy, the latter being the cardinal sign of this pathology. To confirm the diagnosis, a transvaginal ultrasound should be performed, which evidences the low implantation of the placenta. This examination is considered the Standard GOLD since obstetric ultrasound presents many limitations (maternal obesity, fetal situation, etc).

Therefore, the current classification of placenta previa is as follows:

Placenta Normoinserta

More than 5 cm from the Internal Cervical Orifice.

Ecografia de placenta normoinserta
Ultrasound of placenta normoinserta

Placenta Low Insertion

Between 2 and 5 cm of the Internal Cervical Orifice..

Placenta de inserción baja
Low insertion placenta

Marginal placenta previa

Less than 2cm from the Internal Cervical Orifice

Ecografia placenta previa Marginal
Marginal placenta previa ultrasound

Placenta Previa Occlusive or partial

0 cm from the Internal Cervical Orifice.

Ecografia de placenta previa oclusiva o parcial
Ultrasound of occlusive or partial placenta previa

Placenta Previa Total occlusive

Overlap of more than 2 cm.

Ecografia de placenta previa oclusiva total
Ultrasound of total occlusive placenta previa

Placenta previa that persists beyond week 28, has a 20% cause preterm delivery.

What is the Treatment of placenta previa?

The treatment must take into account the degree of bleeding, gestational age, fetal vitality, associated pathologies and whether or not there is labor. Evidence that the treatment can be of two types conservative and of urgency.

However, the objectives of therapy for placenta previa must be taken into account:

  • Preserve the maternal state.
  • Preserve the fetal state.
  • Avoid complications.

Preserve the maternal state

  • End all pregnancy greater than 36 weeks
  • Define the initial hemodynamic state and the estimated blood losses.
  • Classify the degree of bleeding as mild, moderate or severe
  • Immediately start resuscitation, with intravenous fluids
  • Define the need for transfusion
  • Practice a cesarean, regardless of the gestational age, in case of not achieving control of bleeding.

Preserve the fetal state

  • Clearly define the gestational age

Avoid complications

  • All pregnancies that were more than 36 weeks old or if fetal lung maturity should be identified.
  • Order rest in bed until the moment of delivery, if there is control of bleeding.
  • Manage all patients outpatiently as long as bleeding has been controlled, fetal well-being exists and a paramedic is available to transfer the patient to an institution in case of new bleeding.
  • Induce pulmonary maturation with corticosteroids as indicated by the lung maturation protocol
  • Inhibit the uterus if uterine activity occurs, avoiding magnesium sulfate, which has been associated with increased perinatal morbidity.
  • Follow up ultrasound every three weeks, looking for intrauterine growth restriction and signs of placenta accreta.
  • Practice weekly amniocentesis to determine the pulmonary maturation profile, starting at week 34.
  • If the mother is Rh negative, she should receive anti D immunoglobulin.

Conservative management

  • Blood loss less than 250cc.
  • More than 36 weeks of gestation and weighing more than 2500 grams.
  • High level hospital for the management of possible complications.
  • Lung maturation, uterus inhibition, monitoring of fetal well-being, rest, control of hemoglobin / hematocrit.

Emergency management

  • Bleeding greater than 250cc
  • Fetal suffering
  • Cesara of urgency
  • Management of volume losses.

Type of Childbirth

  • If it is not an obstructive placenta, it can induce labor, taking into account the state of the fetus mother binomial, that is, having a protocol for this type of delivery.

Management of hemorrhage in the third stage

  • Uterine massage
  • Oxytocin
  • Curettage of the placental bed
  • Uterine tamponade.
  • Uterine artery embolization.
  • Application of hemostatic sutures.
  • Circular sutures in the placental bed.
  • Uterine or hypogastric ligation
  • Hysterectomy

When is caesarean section indicated in placenta previa?

  • Partial or total placenta previa (TYPE III-IV)
    • Note: Placenta marginal or low insertion must be evaluated according to the protocols of each institution.
  • Intense bleeding
  • Poor maternal state, Hypovolemic shock
  • Signs of fetal distress evidenced in fetal monitoring
  • Associated discrepancies
  • Non-obstructive placentas


Maternal: After prenatal bleeding from placenta previa, maternal hemorrhage, shock and death may occur. The patient may also die as a result of hemorrhage during or after childbirth (Red Code), due to operative trauma, infection, embolism or placenta accreta.

Fetal: Prematurity is the cause of 60% of perinatal deaths. The fetus may die as a result of intrauterine anoxia or injury during birth, infant respiratory distress syndrome, hypothermia, hydroelectrolyte alteration, metabolic disturbance, and increased likelihood of neonatal sepsis. Restriction of intrauterine growth (IUGR).


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