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Placenta Previa

Definition
Attachment of the placenta to the lower segmet of the uterus

Incidence
0.5% of pregnancies
More common in multipara
More common in twin pregnancies due to the large size of the placenta

Aetiology
Low implantation of the blastocyst
Development of the chorionic villi in the decidua capsularis to  leading to attachment to the lower uterine segment
Large placenta as in twin pregnancy

Types
First Degree (Type I / PP lateralis / low lying placenta)
The lower edge of the placenta reaches the lower uterine segment but not the internal os
Second Degree (Type II /PP marginalis)
The lower edge of the placenta reaches the margin of the internal os but does not cover it
Third Degree (Type III / PP incomplete Centralis)
Placenta covers part of the cervical os; the placenta covers the internal os partially dilated but not when it is fully dilated
Fourth Degree (Type IV / PP Complete centralis)
Placenta completely covers the os, even when the cervix is dilated; the placenta covers the internal os completely whether the cervix is partially or fully dilated
Placenta previa marginalis posterior has worse prognosis than marginalis anteriror since it encroaches on the true conugate diameter delaying engagement of the head and engangement of the head will compress the placenta against the sacrum, causing fetal asphxia

Mechanism  of Bleeding
During the third trimester and labour there is progressive stretching of the lower uterine. The segment placenta is non elastic and cannot stretch with the lower segment. So there is separation of a part of the placenta with bleeding. As the labour progresses there is there is more separation of the placenta and more bleeding.

Symptoms and Signs
Causeless, Painless and Recurrent bright red vaginal bleeding.
Bleeding may follow sexual intercourse.
It is painless but labour pains will be there.
Fortunately the first bleeding is usually not severe
Shock may follow if the bleeding is severe
Anemia if the blood loss is recurrent
The uterus size is corresponding to the gestational age, relaxed and non-tender
Fetal parts and FHS are easily detected
Malpresentation like transvers lie  or oblique lie or breech presentation may be present
The head may not engage due to placenta occupying the lower segment
Any PV examination may provoke severe bleeding; can be done only in OT under GA with cross matched blood in hand and when everything is ready for a Caesarian Section : when the index finge is inserted gently the placenta can be felt as a tough fibrous mass

Investigations
USGM : The lower edge of the placenta is less than 3 cm from the margin of the internal os
The posterior placenta is obscured by  the fetus: use head down tilt of the mother or displace the fetus manually to visualize. The distance of the presenting part from the sacral promontory is > 1.5 cm
With increasing gestational age and the formation of the lower uterine segment, a gap develops between the placental edge and the internal os "placental migration". So it is recommended to repeat scan when placenta is diagnosed in mid-pregnancy

Management
Admit when bleeding occurs
No PV
No vaginal pack
Sterile vulval pad permitted
No oral intake allowed as anesthesia may be required
Be ready to combat shock
Get at least two units of blood ready
USGM to differentiate between abruptio placentae and placenta previa
Assess fetal viability age, position and presentation.
Assess the amount of bleeding
Be ready for an emergency CS
If 37 weeks induction or CS

Conservative Treatment
Hopitaliz, bed rest, monitor fetal well being, care of anemia, Anti-D immunoglobulin if Rh-negative mother
If in labour PV with precautions mentioned above
Decide whether to induce labor by amniotomy + oxytocin or CS

Vaginal delivery is allowed if
  1. Plcenta is lateralis or marginalis anterior
  2. Bleeding is very little
  3. Vertex presentation
  4. Adequate pelvis witn no soft tissue obstruction
  5. Partially dilated cervix to allow amniotomy

Caesarean section is indicated in 
Placenta previa centralis whether complete or incomplete even if the fetus is dead.
Placenta previa marginalis posterior
Severe bleeding
Presentation is other than vertex
Other obstetric indications lie contracted pelvis, cord prolapse or elderly  primigravida
Vasa previa

Although upper segment C.Sl is sometimes advocated lower segment C.S.  is prferable because it allows better control of bleeding from the placental site and it leaves a stronger scar; if placenta is anterior it is gently displaced laterally to reach the fetal head or it can be cut through; the latter is not preferred.

Complications of Placenta Previa
Maternal mortality is 0.2%
During pregnancy
  1. PTB
  2. APH
  3. Malpresentation
  4. Non-engagement

During Labour
  1. Premature rupture of membrane
  2. Cord prolapse
  3. Inertia
  4. Obstructed labour
  5. Postpartum hemorrhage
  6. Retained placenta
  7. Placenta accreta - may necessitate hysterectomy
  8. Lacerations of lower segment due to increased vascularity and riability
  9. Air embolism due to low placental site

Foetal
Fetal mortality 20%
Prematurity
Asphyxia
Malformations (2 %)


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