Placenta Previa

written by Reuben Kiggundu, June 2009

Complicates 1:250 pregnancies over weeks

 

Placenta previa occurs when the embryo implants in the lower part of the uterus, towards the cervix. This makes it easy for the placenta to tear, leading to hemmorhage, or block the cervical canal, necessitating Cesarean section.

 

The placenta can be low-lying, marginal, partial or complete, in increasing order of severity.

low-lying: lower margin dips into lower uterine segment, with edge lies within 2-3.5cm of internal cervical os.

marginal: placenta within 2cm of internal os, but doesn’t cover it

partial: placenta covers internal os when closed but not when fully dilated

complete/central: placenta covers internal os when fully dilated.


 

Causes and Risk Factors

 

Risk factors for placenta previa include:

 

Signs, Symptoms, and Diagnosis

 

  • history
  • physical exam
  • lab investigations
  • diagnostic imaging

History

Painless bright red bleeding in the third trimester. Bleeding may be severe and cause symptoms of shock.

Physical Exam

Lab Investigations

  • Apt test to confirm the source of the blood
  • Kleihauer-Betke test
  • APTT,PT
  • Fibrinogen degradation products

Diagnostic Imaging

Because of placenta previa, always do an abdominal ultrasound before doing a vaginal examination in third trimester bleeding. Ultrasound can confirm 95-100% of diagnosis.

 

Differential Diagnosis

The differential diagnosis for bright red bleeding includes:

 

 

 

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Pathophysiology

 

Placenta previa is initiated by implantation of the embryo in the lower uterus. With growth of the placenta the cervical os may become covered by the developing placenta.

 

Bleeding is thought to occur secondary to the thining of the lower uterine segment in preparation for the onset of labor. The placental attachments become disrupted or tear with this thining 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 bleeding are digital vaginal examination and sexual intercourse.

 

 

 

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Treatments

Counsel patients with placenta previa about risk of recurrence and ask them to see an obstetrician early on during the next pregnancy.

 

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Consequences and Course

Placenta Previa

Placenta Previa

written by Reuben Kiggundu, June 2009

Complicates 1:250 pregnancies over weeks

 

Placenta previa occurs when the embryo implants in the lower part of the uterus, towards the cervix. This makes it easy for the placenta to tear, leading to hemmorhage, or block the cervical canal, necessitating Cesarean section.

 

The placenta can be low-lying, marginal, partial or complete, in increasing order of severity.

low-lying: lower margin dips into lower uterine segment, with edge lies within 2-3.5cm of internal cervical os.

marginal: placenta within 2cm of internal os, but doesn’t cover it

partial: placenta covers internal os when closed but not when fully dilated

complete/central: placenta covers internal os when fully dilated.


 

Causes and Risk Factors

 

Risk factors for placenta previa include:

  • multiparity
  • history of placenta previa
  • increasing marternal age
  • previous uterine curettage
  • previous ceaserian section
  • chronic hypertension
  • smoking
  • previous uterine instrumentation

 

Signs, Symptoms, and Diagnosis

 

  • history
  • physical exam
  • lab investigations
  • diagnostic imaging

History

Painless bright red bleeding in the third trimester. Bleeding may be severe and cause symptoms of shock.

Physical Exam

Lab Investigations

  • Apt test to confirm the source of the blood
  • Kleihauer-Betke test
  • APTT,PT
  • Fibrinogen degradation products

Diagnostic Imaging

Because of placenta previa, always do an abdominal ultrasound before doing a vaginal examination in third trimester bleeding. Ultrasound can confirm 95-100% of diagnosis.

 

Differential Diagnosis

The differential diagnosis for bright red bleeding includes:

  • cervicitis
  • premature rupture of membranes
  • preterm labor
  • vaginitis
  • vulvovaginitis

 

 

 

return to top

 

 

Pathophysiology

 

Placenta previa is initiated by implantation of the embryo in the lower uterus. With growth of the placenta the cervical os may become covered by the developing placenta.

 

Bleeding is thought to occur secondary to the thining of the lower uterine segment in preparation for the onset of labor. The placental attachments become disrupted or tear with this thining 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 bleeding are digital vaginal examination and sexual intercourse.

 

 

 

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Treatments

  • IV access, lab work( CBC, group and crossmatch)
  • Establish cardiac and fetal monitoring.
  • Monitor mother’s vitals.
  • Mandatory c/s is required for delivery, but patients may be admitted to hospital for bed and pelvic rest and tocolysis if they are preterm and stable and if the bleeding has stopped.
  • Rhogam.

Counsel patients with placenta previa about risk of recurrence and ask them to see an obstetrician early on during the next pregnancy.

 

return to top

 

 

 

Consequences and Course

  • IUGR
  • Intrauterine fetal death
  • Hemorhage and anemia

50% of women with placenta previa have preterm delivery, which is a major cause of perinatal morbidity and mortality.

 

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Resources and References