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Placenta praevia is an important cause of maternal and fetal morbidity and mortality. Placenta praevia and placental abruption are the most important causes of antepartum haemorrhage, being responsible for more than half of the cases. Antepartum haemorrhage is defined as any vaginal bleeding from the 24th week of gestation until delivery.
Placenta praevia exists when the placenta is inserted wholly or in part into the lower segment of the uterus. Placenta accreta (morbidly adherent placenta) is a rare but important complication of placenta praevia. See the separate Placenta and Placental Problems article.
Placenta praevia is graded by ultrasound findings as:
- Major, if the placenta covers the internal os of the cervix.
- Minor or partial, if the leading edge is in the lower segment but not covering the os.
The incidence is rising with the increasing caesarean section rate. The overall incidence is 1/200 births, and 1/1,000 are major with placenta over the entire cervix.
- Previous history of placenta praevia.
- Previous caesarean section:
- Relative risk after one caesarean section increased by 2.2.
- Relative risk after two caesarean sections increased by 4.1.
- Relative risk after three caesarean sections increased by 22.4.
- Advancing maternal age.
- Increasing parity.
- Cocaine use during pregnancy.
- Previous spontaneous or induced abortion.
- Deficient endometrium due to past history of, for example, endometritis, manual removal of placenta, curettage.
- Assisted conception.
- It may be an incidental finding on routine anomaly ultrasound.
- Painless bleeding starting after the 28th week (although spotting may occur earlier) is usually the main sign:
- Typically, it is sudden and profuse but usually does not last for long and so is only rarely life-threatening.
- Women with placenta praevia are reported to be 14 times more likely to bleed in the antenatal period compared with women without placenta praevia.
- There may be some initial pain in approximately 10% of cases with coincidental placental abruption.
- There is a high risk of preterm delivery; in 25% of cases, spontaneous labour appears in the subsequent few days.
- In a small proportion of cases, less dramatic bleeding occurs or does not start until spontaneous rupture of membranes or onset of labour.
- High presenting part or abnormal lie; it may be impossible to push the high presenting part into the pelvic inlet. In 15% of cases the fetus presents in an oblique or transverse lie.
- Usually, there is no indication of fetal distress unless complications occur.
Clinical suspicion should be high in any woman with vaginal bleeding after 20 weeks of gestation. Irrespective of previous imaging results, a high presenting part, an abnormal lie and painless or bleeding provoked by sexual intercourse are highly suggestive of a low-lying placenta but may not be present. The definitive diagnosis relies on determining the site of the leading edge of the placenta on ultrasound imaging:
- This may be low at the 20-week scan but 'apparent' migration occurs during the second and third trimesters, due to the development of the lower uterine segment:
- Migration is less likely if the placenta is posterior or if there has been a previous caesarean section.
- A review of 714 women found that, even with a partial praevia, there was a 50% chance of persistence leading to a caesarean delivery if there had been a previous uterine scar, compared with 11% chance if there was no scar.
- Even in women with overlap of 25 mm, migration is still possible
- Transvaginal scans (TVS) improve the accuracy of placental localisation and are safe, so the suspected diagnosis of placenta praevia at 20 weeks of gestation by abdominal scan should be confirmed by TVS.
- Ultrasound cannot exclude a placental abruption which is a clinical diagnosis.
Other investigations will depend on context but may include FBC, group and cross-match, fetal monitoring.
Screening and follow-up
A further TVS is recommended for all women whose placenta reaches or overlaps the cervical os at their anomaly scan as follows:
- Women who bleed should be managed individually according to their needs.
- In cases of asymptomatic suspected minor praevia, follow-up imaging can be left until 36 weeks.
- In cases with asymptomatic suspected major placenta praevia, a TVS should be performed at 32 weeks, to clarify the diagnosis and allow planning for third-trimester management and delivery.
Minor placenta praevia
- A woman with a minor placenta praevia may be able to deliver vaginally.
- A placental edge less than 2 cm from the os has been suggested as indicating a need for delivery by caesarean section, especially if it is posterior or thick.
- If the head has engaged at the time of a planned caesarean section there may be a role for TVS as the lower segment continues to develop after 36 weeks.
- NB: if the placenta is anterior, is reaching the os and the woman has previously had a caesarean section, she should be managed as if she has placenta accreta (see 'care bundle', below).
Major placenta praevia
- Major placenta praevia will require delivery by caesarean section.
- Women should be advised not to have penetrative intercourse.
- There is no place for routine tocolytics to prevent bleeding.
- There is insufficient evidence to recommend cervical cerclage to prolong pregnancy or prevent bleeding.
- Women with major placenta praevia who remain asymptomatic, ie no bleeding, require careful counselling before contemplating outpatient care once they have reached 34 weeks of gestation.
- Women who have experienced a bleed, should be encouraged to stay in hospital from 34 weeks of gestation:
- Any home-based care requires close proximity to the hospital, the constant presence of a companion and fully informed consent from the woman.
- Any woman being managed at home should attend hospital immediately if she experiences any bleeding, any contractions or any pain (including vague suprapubic period-like aches).
- Where possible, elective caesarean section should be deferred to 38 weeks to minimise neonatal morbidity (36-37 weeks if placenta accreta is suspected). However, the benefits of additional maturity need to be weighed against the risk of major haemorrhage and the possibility that repeated small haemorrhages may cause intrauterine growth restriction.
Placenta praevia with a history of caesarean section (placenta accreta suspected)
- There is a high risk of placenta accreta (morbidly adherent placenta) in women with placenta praevia who have previously had a caesarean section.
- Antenatal imaging techniques (eg, grey scale/colour flow Doppler/3D ultrasonography) can help to raise the suspicion of a morbidly adherent placenta and should be considered in any situation where any part of the placenta lies under the previous caesarean section scar even if not praevia; however, the definitive diagnosis can be made only at surgery
- In response to findings of the Confidential Enquiry into Maternal Deaths, a care bundle for suspected placenta accreta has been developed with six elements of good care:
- Consultant obstetrician planned and directly supervising delivery.
- Consultant anaesthetist planned and directly supervising anaesthetic at delivery.
- Blood and blood products available on site.
- Multidisciplinary involvement in pre-operative planning.
- Discussion and consent including possible interventions - eg, hysterectomy, leaving the placenta in place.
- Locally available level 2 critical care bed.
- The uterus should be opened at a site distant from the placenta, delivering the baby without disturbing the placenta. This allows either conservative management of the placenta or hysterectomy if accreta is confirmed.
- Between 2009-2012 there was one death from placenta praevia in a woman with placenta praevia percreta, in which the placenta breeches the uterine myometrium. There were no deaths from unexpected placenta praevia, suggesting adherence to the care bundle is having an effect, as there had previously been, on average, one death per year.
See the separate Antepartum Haemorrhage article.
Admit the patient to hospital.
DO NOT PERFORM A VAGINAL EXAMINATION, as this may start torrential bleeding in the presence of placenta praevia.
- Blood loss is assessed and cross-matched for possible transfusion.
- Resuscitation if indicated; the mother is the priority and should be stabilised prior to any assessment of the fetus.
- Appropriate surgical intervention may be required:
- In severe bleeding the baby is delivered urgently whatever its gestational age.
- Hysterectomy should also be considered in severe cases.
- If immediate delivery is not likely, maternal steroids may be indicated in order to promote fetal lung development and reduce the risk of respiratory distress syndrome and intraventricular haemorrhage.
- Potentially fatal hypovolaemic shock resulting from severe antepartum, intrapartum or postpartum bleeding.
- Venous thromboembolism is associated with prolonged inpatient care and the hazards of prophylactic anticoagulation in women at high risk of bleeding.
- Rare: placenta accreta, increta and percreta
- Fetal haemorrhage, prematurity, intrauterine asphyxia or birth injury.
- A prospective study of 328 European women demonstrated the high maternal and neonatal morbidity associated with placenta praevia:
- 42.3% antepartum haemorrhage.
- 7.1% postpartum haemorrhage.
- 30% maternal anemia.
- 4% co-existing placenta accreta.
- 5.2% hysterectomy.
- 54.9% preterm birth.
- 35.6% low birth weight <2500 g.
- 1.5% fetal mortality.
- A population-based study in the USA demonstrated that neoatal mortality is increased 4.3-fold: it is 10.7 per 1,000 births when there is placenta praevia compared with 2.5 per 1,000 where there is not.
- Maternal mortality secondary to haemorrhage is 0.49 per 100,000 maternities in the UK: there were no reported deaths between 2009-2012 due to placenta praevia; there was one death due to placenta praevia percreta.
Further reading and references
; Royal College of Obstetricians and Gynaecologists (September 2018)
; Ante-partum haemorrhage: an update. J Obstet Gynaecol. 2008 May28(4):377-81.
; Interventions for suspected placenta praevia. Cochrane Database Syst Rev. 2003(2):CD001998.
; Etiology and risk factors for placenta previa: an overview and meta-analysis of observational studies. J Matern Fetal Neonatal Med. 2003 Mar13(3):175-90.
; Royal College of Obstetricians and Gynaecologists (December 2011)
; NICE Clinical Guideline (March 2008, updated 2017)
; MBRRACE-UK, Dec 2014
; Royal College of Obstetricians and Gynaecologists (October 2010)
; Placenta praevia: incidence, risk factors and outcome. J Matern Fetal Neonatal Med. 2015 Jun 4:1-4.
; The effect of placenta previa on neonatal mortality: a population-based study in the United States, 1989 through 1997. Am J Obstet Gynecol. 2003 May188(5):1299-304.
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