Placenta Praevia
RCOG Placenta Praevia and Placenta Accreta: Diagnosis and Management (Green-top Guideline No. 27a). Last reviewed: Sep 2018.
Background Information
Definition
Placenta praevia is a condition where the placenta lies directly over, and covers the internal cervical os.
Normal: placental edge is at least 2cm away from the internal cervical os.
Low-lying placenta: placental edge is <2cm away from the internal cervical os, but does NOT cover it (different from placenta praevia).
Aetiology
The aetiology of placenta praevia relates to the placenta initially implanting and developing within the lower uterine segment.
Risk Factors
| Cateogry | Specific risk factors |
|---|---|
| Lifestyle and environmental factors |
|
| Prior obstetric history |
|
| Current pregnancy factors |
|
Clinical Features
Many patients are asymptomatic
If symptomatic:
- Classically painless vaginal bleeding at >24 weeks (antepartum haemorrhage)
- Soft, non-tender uterus on palpation
- Bleeding could be recurrent throughout the 3rd trimester
Placenta praeia is often associated with high-presenting fetal part or fetal malpresentation (see the Fetal Presentation and Malpresentation article for more information).
Note that the traditional teaching is placenta praevia = painless vaginal bleeding.
RCOG emphasises that patients may present with vague, suprapubic period-like aches, along with vaginal bleeding.
Complications
- Massive haemorrhage (both antenatal and postpartum haemorrhage)
- Pre-term delivery (due to the risk of sudden severe bleeding)
- ↑ Risk of placenta accreta spectrum (see the Placenta Accreta Spectrum article)
There is a high rate of placenta accreta spectrum and placenta praevia co-existence.
This is because placenta praevia strongly predisposes the development of placenta accreta spectrum. However, the reverse is not true (i.e. plaenta accreta spectrum does NOT predispose the development of placenta praevia).
Diagnosis
Screening
As part of the routine antenatal care / timetable, an anomaly scan (TAUS) is offered at 18-21 weeks to screen for placenta praevia
- The scan is also used to screen for 11 structural conditions (see the Fetal Anomaly Screening Programme for more information)
- If the TAUS identified the woman as at risk (suspected low-lying placenta / placenta praevia) → further imaging is necessary (see the diagnosis section below)
Note that a trans-abdominal ultrasound scan is NOT diagnostic, it’s primary purpose is to identify those at risk of placenta praevia.
Diagnosis
Confirmatory test: TVUS
- Perform at 32 weeks to confirm diagnosis
- If the 32-week scan identified placenta praevia → repeat at 36 weeks (for delivery planning)
Rationale for needing 2 follow-up TVUS
- The lower uterine segment develops and stretches during the third trimester, which causes an apparent placental “migration” that resolves a low-lying placenta in 90% of cases before term (thus the 32-week scan)
- Even if placental praevia is confirmed at the 32-week scan, ~50% cases of placenta praevia will resolve (thus the 36-week scan)
Shift in terminology:
- Current guidelines explicitly recommend discontinuing the use of the older sub-classifications (minor, marginal, partial, and complete praevia based on how much of the cervical os was covered).
- Latest guidelines recommend that once the placenta lies directly over the internal os, it should be classified strictly as ‘placenta praevia’, with no further subclasification.
- Ultrasound reporting is now limited to just 3 distinct categories:
- Normal – placental edge ≥2 cm away from the internal os
- Low-lying placenta – placental edge <2 cm away from the internal os
- Placenta praevia – placenta lies directly over the internal os
Management
Placenta Praevia
IMPORTANT: if a patient with placenta praevia goes into spontaneous labour → emergency Caesarean section is indicated (as it can rapidly lead to massive life-threatening haemorrhage)
Elective mode of delivery depends on whether the patient is symptomatic or not:
| Symptoms? | Delivery plan |
|---|---|
| Symptomatic (history of vaginal bleeding) | Timing: planned late pre-term delivery between 34-36 weeks
Mode: Caesarean section is necessary (vaginal delivery is NOT recommended) |
| Asymptomatic (no history of vaginal bleeding at all) | Timing: planned delivery between 36-37 weeks
Mode: Caesarean section is necessary (vaginal delivery is NOT recommended) |
If a woman is being treated at home, she must be explicitly instructed to attend the hospital immediately if she experiences any bleeding, spotting, contractions, or pain (including vague suprapubic aches).
Low-Lying Placenta
Offer patients either of the following mode of delivery:
- Planned Caesarean section, or
- Vaginal delivery
The decision should be individualised, depending on the patient’s clinical background and ultrasound findings.
If a patient with a low-lying placenta goes into spontaneous labour, an emergency Caesarean section is NOT absolutely necessary.