Placenta praevia is where the placenta is attached in the lower portion of the uterus, lower than the presenting part of the fetus. Praevia directly translates from Latin as “going before”.
The RCOG guidelines (2018) recommend the following definitions:
- Low-lying placenta is used when the placenta is within 20mm of the internal cervical os
- Placenta praevia is used only when the placenta is over the internal cervical os
Placenta praevia occurs in around 1% of pregnancies. It is a notable cause of antepartum haemorrhage.
TOM TIP: The three causes of antepartum haemorrhage to remember are placenta praevia, placental abruption and vasa praevia. These are serious causes with high morbidity and mortality. Causes of spotting or minor bleeding in pregnancy include cervical ectropion, infection and vaginal abrasions from intercourse or procedures.
Risks
Placenta praevia is associated with increased morbidity and mortality for the mother and fetus. The risks include:
- Antepartum haemorrhage
- Emergency caesarean section
- Emergency hysterectomy
- Maternal anaemia and transfusions
- Preterm birth and low birth weight
- Stillbirth
Grades
Traditionally, there are four grades of placenta praevia. You may still come across these in textbooks and exams:
- Minor praevia, or grade I – the placenta is in the lower uterus but not reaching the internal cervical os
- Marginal praevia, or grade II – the placenta is reaching, but not covering, the internal cervical os
- Partial praevia, or grade III – the placenta is partially covering the internal cervical os
- Complete praevia, or grade IV – the placenta is completely covering the internal cervical os
The RCOG guidelines (2018) recommend against using this grading system, as it is considered outdated. The two descriptions used are low-lying placenta and placenta praevia.
Risk Factors
The risk factors for placenta praevia are:
- Previous caesarean sections
- Previous placenta praevia
- Older maternal age
- Maternal smoking
- Structural uterine abnormalities (e.g. fibroids)
- Assisted reproduction (e.g. IVF)
Presentation and Diagnosis
The 20-week anomaly scan is used to assess the position of the placenta and diagnose placenta praevia.
Many women with placenta praevia are asymptomatic. It may present with painless vaginal bleeding in pregnancy (antepartum haemorrhage). Bleeding usually occurs later in pregnancy (around or after 36 weeks).
Management
For women with a low-lying placenta or placenta praevia diagnosed early in pregnancy (e.g. at the 20-week anomaly scan), the RCOG guideline (2018) recommends a repeat transvaginal ultrasound scan at:
- 32 weeks gestation
- 36 weeks gestation (if present on the 32-week scan, to guide decisions about delivery)
Corticosteroids are given between 34 and 35 + 6 weeks gestation to mature the fetal lungs, given the risk of preterm delivery.
Planned delivery is considered between 36 and 37 weeks gestation. It is planned early to reduce the risk of spontaneous labour and bleeding. Planned cesarean section is required with placenta praevia and low-lying placenta (<20mm from the internal os).
Depending on the position of the placenta and fetus, different incisions may be made in the skin and uterus, for example, vertical incisions. Ultrasound may be around the time of the procedure to locate the placenta.
Emergency caesarean section may be required with premature labour or antenatal bleeding.
The main complication of placenta praevia is haemorrhage before, during and after delivery. When this occurs, urgent management is required and may involve:
- Emergency caesarean section
- Blood transfusions
- Intrauterine balloon tamponade
- Uterine artery occlusion
- Emergency hysterectomy
Last updated September 2020