Introduction

Placenta praevia describes the most common a type of abnormal placental location in which the placenta overlies the cervical os. This complication of pregnancy affects approximately 0.5% of all pregnancies with up to 90% of cases resolving spontaneously before becoming symptomatic, however incidence is rising.

The condition is a major cause of maternal and fetal mortality and morbidity. Placenta praevia is a cause of antepartum haemorrhage, vaginal bleeding in the 3rd trimester and indication for caesarean section. After a diagnosis is confirmed with ultrasonography, placenta praevia is managed with supportive therapies and caesarean section.

Classification

The classification of placenta praevia is based on the relationship and distance between the placenta and the cervical os. Currently, the Royal College of Obstetricians and Gynaecologists (RCOG) recommend the use of the American Institute of Ultrasound in Medicine (AIUM) classification of placenta praevia.

There are 4 grades:
  • Grade 1 - also known as a low lying placenta
    • The placenta is in the lower uterine segment
    • The lower edge of the placenta is 0.5-2cm from the internal cervical os
  • Grade 2 - also known as marginal praevia
    • The lower edge of the placenta reaches the internal cervical os
    • The placenta extents to the margin of the os but does not cover it
  • Grade 3 - also known as partial praevia
    • The placenta partially covers the internal cervical os
  • Grade 4 - also known as complete praevia
    • The placenta completely covers the internal cervical os

Prognosis worsens with higher grades. Low lying placentas are found in 4-6% of pregnancies before 20 weeks, however up to 90% of these spontaneously resolve as the uterus grows and the placenta migrates upwards relative to the cervical os. On the other hand, only 10% of complete praevias resolve. Unresolved cases will require caesarean delivery to prevent maternal and fetal mortality.

Epidemiology

  • Incidence: 20.00 cases per 100,000 person-years
  • Peak incidence: 30-40 years
Condition Relative
incidence
Placenta praevia1
Placental abruption1.00
<1 1-5 6+ 16+ 30+ 40+ 50+ 60+ 70+ 80+

Aetiology

The cause of placenta praevia is not fully understood.

However, it is likely to be the result of a combination of factors, the most important of which being previous uterine scarring. This scarring is typically the result of cesarean section, but other causes are possible. This scarring leads to the placenta implanting into the lower uterine segment, although the mechanism is not known.

Other factors contributing to the development of placenta praevia include:
  • Older maternal age
  • Multiple pregnancies
  • A large number of previous pregnancies
  • Smoking

Pathophysiology

Placenta praevia occurs when the blastocyst, which later develops into the placenta, implants into the lower uterine segment. This can result in the placenta overlying the cervical os and obstructing the pathway of the fetus during labour.

Bleeding in placenta praevia occurs for a few key reasons:
  • As the placenta is in a vulnerable location, bleeding can be triggered by pressure around the cervix causing minor placental trauma
    • Penetrative sexual intercourse
    • Vaginal examination.
  • Labour and delivery
    • The placenta is at risk of rupture because it is blocking the path of the fetus
    • This may results in massive haemorrhage from both the mother and fetus, leading to death

Clinical features

Placenta praevia is often identified before symptoms develop during a routine ultrasound appointment. When symptoms do present, they typically occur in the 3rd trimester of pregnancy.

The main clinical feature of placenta praevia is painless vaginal bleeding.
  • Severity of bleeding can range from spotting to life-threatening haemorrhage
  • Mean gestational age of initiation of bleeding is 30 weeks, however 1/3 patients present before this

On examination, there should be no evidence of a vaginal or cervical cause of the bleeding and the uterus is not typically painful, unless in labour. It is worth noting that vaginal and rectal examinations are not recommended in placenta praevia. Intercourse should be avoided too. The presence of risk factors may also be identified.

Risk factors for placenta praevia include:
  • Multiparity and multiple pregnancies
  • Uterine scarring
    • Typically as a result of caesarean delivery
    • Other causes include uterine rupture, endometriosis and dilation and curettage procedures
  • Previous placenta praevia (increases risk to 4-8% of subsequent pregnancies)
  • Advanced maternal age


Investigations

When placenta praevia is suspected, ultrasonography is used to definitively diagnose the condition. Transabdominal ultrasound detects placenta praevia in approximately 95% of cases, whereas transvaginal ultrasound detects the condition in almost 100% of patients.

Typically, placenta praevia is detected around 20 weeks during a routine anomaly scan. Although most of these cases will spontaneously resolve, a follow-up transvaginal ultrasound at 32 weeks is indicated to identify the unresolved cases. Patients with persistent, but asymptomatic low-lying placenta or placenta praevia at 32 weeks should be followed up at 36 weeks for a subsequent transvaginal ultrasound to plan delivery.

Depending on the severity of bleeding further investigations should be considered:
  • Full blood count (FBC) to identify maternal anaemia or platelet disorder
  • Clotting studies may aid in identifying bleeding disorders, but are only indicated if the platelet count is abnormal
  • Blood type and cross-match will be required in preparation for surgery
  • The Kleihauer test should be used in rhesus-negative mothers to determine the dose of anti-D required
  • Fetal cardiotocography (CTG) is recommended for patients admitted to hospital to assess fetal wellbeing and aid in decision making
  • Biochemistry such as liver function tests and urea & electrolyte levels. This test can help rule out hypertensive disorders such as pre-eclampsia

Differential diagnosis

As an important cause of antepartum haemorrhage (bleeding after 28 weeks gestation), the differentials for placenta praevia include other causes of this bleeding and other placenta abnormalities.

Placental abruption occurs when the placenta detaches from the endometrium and results in haemorrhage at the site of detachment. Like placenta praevia, the volume of bleeding is incredibly variable and fetal compromise can occur in severe cases. Differentiating factors include:
  • Placental abruption is typically painful, whereas placenta praevia is typically painless
  • The uterus is more likely to be tense on examination
  • Ultrasound assessment will show separation of the placenta from the uterine wall, however placenta praevia and abruption can co-exist in the same patient

Miscarriage is an important differential and one that may frighten patients most. Differentiating factors include:
  • Bleeding in some cases of miscarriage are accompanied by expulsion of products of conception
  • The bleeding in miscarriage often occurs alongside cramp-like abdominal pain, where placenta praevia is typically painless
  • Miscarriage is more common in the 1st and 2nd trimesters - placenta praevia often causes symptoms in the 3rd trimester
  • Examination signs of miscarriage may be present:
    • Cervical os may be open
    • Identification of products of conception
    • Uterine changes
  • Miscarriage is generally diagnosed clinically, however ultrasound can be helpful in differentiating cases

Other placental abnormalities similar to placenta praevia include placenta accreta. This condition describes when the placenta invades too deeply into the uterine wall. It causes variable bleeding in the 3rd trimester, is more common in those with uterine scarring and can be life threatening. Ultrasound is the most effective way of differentiating these two conditions.

Local causes of antepartum haemorrhage should be considered too. These include genital lesions (benign and malignant), genital lacerations and trauma, cervical ectropion and local infections. Most of these should be ruled out with thorough history and examination. Vaginal should be taken in minor bleeding to exclude infection.

Management

In the case of serious haemorrhage, patients should be managed with an ABCDE approach with an aim to stabilise the patient. In addition to following standard resuscitation guidelines, these should be considered:
  • Corticosteroids should be considered in cases where the gestational age is below 34 weeks
  • Tocolytics may be used to delay labour to provide maximum benefit from corticosteroids therapy
  • Anti-D should be given with 72 hours of onset of bleeding in any resus negative mother

Cesarean section is often necessary in major forms of placenta praevia and in acute haemorrhage. They may also be indicated if attempts to delay labour have failed. Prophylactic and intra-operative antibiotics are recommended.

Patients identified through routine screening at 20 weeks should have a follow-up ultrasound scan at 32-34 weeks to confirm praevia, as many patients will spontaneously resolve.
  • Recommend pelvic rest
    • No penetrative intercourse
    • No vaginal douching
    • Avoid vaginal examination unless completely necessary
  • Corticosteroids therapy should be considered in all patients

When presenting with labour, patients with minor placenta praevia (e.g. low lying placenta) should be monitored closely. These patients can deliver normally, however there is an increase risk of a requiring cesarean delivery. Major forms of placenta praevia typically have a cesarean delivery arranged for 37-38 weeks gestation.

Complications

The complications of placenta praevia generally arise from the risks presented by haemorrhage, pre-term birth and those of cesarean delivery.

Haemorrhage in placenta praevia can be incredibly variable, however minor bleeding is less likely to result in major complications. Any amount of blood loss can result in maternal anaemia. The risk increases with the severity of bleeding and low pre-bleed maternal haemoglobin and iron stores.

Patients are at higher risk of:
  • Pre-term birth
    • Patients can present with early labour or require emergency c-section before the delivery date due to bleeding
    • In major placenta praevia patients may be booked for an elective c-section before the due date to reduce risk
    • The complications for the fetus are the same for other pre-term birth, however there is a higher risk of anaemia
  • Fetal death
  • Intrauterine growth restriction

Patients who undergo cesarean section for placenta praevia are at an increased risk of bleeding that requires blood products and hysterectomy.
  • Hysterectomy may be indicated due to acute haemorrhage and has been associated with other placenta abnormalities, such as placenta accreta.