Placenta praevia

Placenta praevia is a complication in pregnancy where the placenta is in front of the internal cervical os. It is a cause of antepartum haemorrhage after 24 weeks of gestation that is important to identify as it is associated with serious risk of morbidity and mortality for the mother and baby alike. It occurs in about 0.5% of pregnancies globally.

🔢 Pathophysiology and classification

The blastocyst implants itself into the uterus about 6 days after fertilisation. It most commonly implants in the fundus of the uterus. Most cases of placenta praevia is most probably attributable to normal variation. The blastocyst cannot implant into scar tissue, therefore the presence of scar tissue is a risk factor that can increase the risk of implantation towards the lower segment of the uterus. The placenta, if overlying the cervix, can become torn or may abrupt (separate from the uterus) which may lead to major haemorrhage and death of both mother and baby.

We can classify the placental location by its relation to the internal cervical os:

1. Normal placental location - the edge of the placenta is >20mm from the internal cervical os.
2. Low-lying placenta - the edge of the placenta is <20mm from the internal cervical os.
3. Placenta praevia - the placenta overlies and covers the internal cervical os.

There is also a classical grading system for placenta previa which classifies it from grades 1-4:

1. Grade I (minor praevia) - lower edge in the lower uterine segment.
2. Grade II (marginal praevia) - lower edge reaching the internal os.
3. Grade III (partial praevia) - placenta partially covers the cervix.
4. Grade IV (complete praevia) - placenta completely covers the cervix.

⚠️ Risk factors

😷 Presentation

⭐️ The classical presenting feature of placenta praevia is painless vaginal bleeding occurring in later pregnancy (after 24 weeks). The amount of vaginal bleeding can vary from spotting all the way to major haemorrhage. The bleeds are usually small before a major haemorrhage.

Other features that suggest placenta praevia could be:

• Non-tender uterus
• Malpresentation of the foetus

🔍 Investigation

🥇 Routinely we have a 20 week anomaly scan to assess for any anomalies in the foetus as well as the placenta. This is done via abdominal ultrasound.

🏆 Transvaginal ultrasound is used to confirm the diagnosis.

If there is a case picked up at the 20 week scan, then we should do a transvaginal ultrasound at:

1. 32 weeks
2. 36 weeks

🧰 Management

🧰

Managing placenta praevia:

• An ultrasound should be done around the time of delivery to identify the location of the placenta. Classical (vertical) incisions may be used, although a transverse incision is still viable and preferred.
• Corticosteroids are given between 34-36 weeks for lung maturity in case of preterm delivery.
• A trial of labour may be done if the edge of the placenta is >20mm from the internal os. However, if haemorrhage develops or there is foetal distress → C-section.

Women with placenta praevia or a low-lying placenta should be have an early planned C-section between 37-38 weeks. However, they will need an emergency C-section if:

1. Spontaneous labour
2. Antepartum haemorrhage
1. For any episode of antepartum haemorrhage we should give anti-D if the mother is Rhesus negative.
2. If we have any antepartum haemorrhage with an unknown placental position then we should follow an ABC approach and stabilise the mother with tranexamic acid, transfusions (depending on the Hb level) before doing an urgent ultrasound. If it is not possible to control the bleeding then an emergency C-section should be performed.

🚨 Complications

• Haemorrhage - this is the main complication that can happen. It may be antepartum haemorrhage, intrapartum haemorrhage or postpartum haemorrhage. Management of haemorrhages require emergency C-section and may warrant blood transfusions, intrauterine balloon tamponade, uterine artery embolisation or even emergency hysterectomy.
• Preterm birth
• Placenta accreta spectrum