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Placenta praevia

Created: 24/5/2006
Updated: 26/9/2006

Placenta praevia

Placenta praevia exists when the placenta is inserted wholly or in part into the lower segment of the uterus. The incidence of praevia is about 0.5% at term and occurs more commonly in mothers who have previously delivered by caesarean section. The diagnosis has evolved from the original clinical I-IV grading system and is determined by ultrasonic imaging techniques relating the leading edge of the placental to the cervical os.

Grade I       Low lying placenta
Grade II     Meets the edge of the os
Grade III    Partially covers the os
Grade IV     Completely covers the os

The anterior or posterior placement of the placental should also be noted


Painless bleeding is the usual presentation, with the first bleed at 27 –32 weeks gestation.


Ultrasound scan should be performed urgently to determine the position of the placenta. Transvaginal ultrasound is more accurate than transabdominal ultrasound in locating the placenta and safe in the presence of placenta praevia.


If there is recurrent bleeding the mother is usually kept in hospital with cross-matched blood readily available. Inpatients are at increased risk of thromboembolic disease and the necessary precautions should be taken. Minor bleeds are managed conservatively but the timing of emergency surgery will be influenced by individual circumstances but, where possible, elective caesarean section should be deferred to 38 weeks to minimise neonatal morbidity.

Blood should be available in the peripartum period. The amount of cross matched blood required will depend on the clinical features of each individual case and the local blood bank services available. If atypical antibodies are present, direct communication and specific plans should be made with the local blood bank.

Any woman going to theatre with known placenta praevia should be delivered by the most experienced obstetrician and anaesthetist on duty. As a minimum requirement during a planned procedure, a consultant obstetrician and anaesthetist should be present within the delivery suite. A junior doctor should not be left unsupervised when caring for these women.
When an emergency arises, consultant staff should be alerted and should attend as soon as possible. Resuscitation follows basic principals of ABC, two 14G cannulas, iv fluids and emergency cross matching blood. Occasionally it may be necessary for the surgeon to divide an anterior placenta praevia in order to gain access to the foetus and this is accompanied by very heavy blood loss.

The mode of anaesthesia for caesarean section depends upon many factors. A senior anaesthetist must be present. A regional technique may be appropriate for a posterior placenta with no active bleeding and no previous sections but this must first be discussed with the obstetrician performing surgery. An anterior placenta especially in cases of previous section or active bleeding should have a general anaesthetic.

Patients with placenta praevia who previously underwent a Caesarean section, are at increased risk of placenta accrete, increta or percreta and torrential haemorrhage. The risk of placenta accreta increases with the number of previous caesarean deliveries.

Placenta accreta is a placenta that is morbidly adherent to the myometrium

Placenta increta is one that invades the myometrium

Placenta percreta is one that penetrates the myometrium

When placenta accreta is thought to be likely, consultant anaesthetic and obstetric input are vital in planning and conducting the delivery. Crossed matched blood should be available and colleagues from other specialties/ subspecialties may be alerted to be on standby to attend as needed.

In the case of placenta accreta, increta and percreta, the risk of haemorrhage, transfusion and hysterectomy should be discussed with the patient as part of the consent procedure.

The most recent Confidential Enquiry into Maternal Deaths in the UK stresses that all caesarean sections performed in women with placenta praevia who have had a previous caesarean section should be conducted by a consultant obstetrician, because of the high risk of major morbidity.


[i] RCOG: Placenta praevia; diagnosis and management

[ii] Management of obstetric hemorrhage; Semin Perinato;l 2003 Feb;27(1):86-104.

[iii] ABC of labour care: Obstetric emergencies;BMJ 1999;318:1342-1345 ( 15 May )

[iv] Perinatal Review Obstetric Emergencies

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