Placenta Previa

Summary

Placenta praevia is implantation of the placenta in the lower uterine segment, partially or completely covering the cervical os, and a major cause of painless antepartum haemorrhage after 24 weeks. Risk is increased with previous caesarean section, multiparity, and advanced maternal age. Diagnosis is confirmed by transvaginal ultrasound, and management depends on stability and gestation, with caesarean section required in most cases. It carries risks of massive haemorrhage, prematurity, and placenta accreta spectrum.

Definition

Placenta praevia refers to implantation of the placenta in the lower uterine segment, where it partially or completely covers the internal cervical os. It is a major cause of antepartum haemorrhage (APH), defined as bleeding from the genital tract after 24 weeks of gestation and before delivery. A low-lying placenta (within 2 cm of the internal os between 24–28 weeks) may migrate upwards, whereas persistence beyond 28 weeks is termed placenta praevia.

Epidemiology

Placenta praevia occurs in approximately 0.5% of pregnancies. The incidence rises with increasing rates of caesarean section and advanced maternal age.

Pathophysiology

The lower uterine segment stretches during late pregnancy and labour, but the placenta lacks elasticity. This stretching can cause separation of placental tissue, leading to recurrent, painless vaginal bleeding. The bleeding is maternal in origin, and can be spontaneous or provoked (e.g., intercourse, vaginal examination).

Classification

Placenta praevia is classified according to the degree of coverage of the internal cervical os:

• Grade I (Low-lying placenta): within 2 cm of the os but not reaching it.

• Grade II (Marginal): reaching the margin of the os without covering it.

• Grade III (Partial/Incomplete centralis): partially covers the os.

• Grade IV (Complete centralis): completely covers the os.

Traditionally, minor (Grades I–II anterior) could allow vaginal delivery, whereas major (Grades II posterior–IV) required caesarean section. Currently, caesarean delivery is preferred for all placenta praevia cases.

Risk Factors

• Previous caesarean section (risk increases with number of scars: 1/160 after 1 CS; 1/10 after 4 CS)

• Multiparity

• Advanced maternal age (>35–40 years)

• Multiple pregnancy

• Previous placenta praevia

• Uterine surgery (e.g., curettage, myomectomy)

• History of uterine infection

• Assisted conception (e.g., IVF)

• Smoking

Clinical Features

• Classical presentation: sudden, painless, bright-red vaginal bleeding in the second half of pregnancy.

• Bleeding is often recurrent and unpredictable.

• Uterus is soft and non-tender; fetal parts are easily palpable.

• Fetal lie may be abnormal (e.g., breech, transverse).

• Bleeding may be incidental finding on routine ultrasound.

Complications: preterm delivery, intrauterine growth restriction (IUGR), malpresentation, premature rupture of membranes (PROM), vasa praevia, and morbidly adherent placenta (accreta, increta, percreta).

Diagnosis

• Transvaginal ultrasound: gold standard; safe and most accurate for placental localisation.

• Transabdominal ultrasound: initial screening.

• Avoid digital vaginal examination until placenta praevia is excluded, as it may cause catastrophic haemorrhage.

Differential Diagnosis of APH

• Placental abruption: painful bleeding, tense/woody uterus, possible fetal distress.

• Vasa praevia: bleeding after membrane rupture, associated with fetal distress.

• Uterine rupture: typically in labour with prior uterine surgery.

• Local genital causes: cervical ectropion, polyps, carcinoma, infection.

• Show of labour: most common, benign cause.

Investigations

• CBC, coagulation profile, blood group and crossmatch (≥4 units), renal and liver function tests.

• Kleihauer test in Rh-negative women to guide anti-D administration.

• CTG ≥26 weeks to assess fetal wellbeing.

Management

General Principles

• Hospital admission for all cases after diagnosis.

• Two wide-bore IV cannulae, blood cross-match, and resuscitation if bleeding is significant.

• Correct maternal anaemia and prepare for possible transfusion.

• Strict pelvic rest: no intercourse, no vaginal examination.

• Administer anti-D within 72 hours for Rh-negative women.

Asymptomatic / Incidental Finding

• Minor/low-lying placenta: repeat scan at 32–36 weeks; many cases resolve spontaneously as pregnancy progresses.

• Major placenta praevia: plan delivery by elective caesarean section at 36–38 weeks.

Symptomatic (Bleeding)

• Stable + preterm: expectant management, corticosteroids for fetal lung maturity, NICU preparation.

• Stable + term: delivery, usually by caesarean section.

• Unstable (maternal or fetal distress): emergency caesarean regardless of gestational age.

• Mild bleeding: conservative inpatient management with close monitoring.

• Massive haemorrhage: aggressive resuscitation, blood products, and urgent delivery.
  
  

Prognosis

Maternal mortality is now rare with modern obstetric care, but placenta praevia carries significant risks of massive haemorrhage, hysterectomy, prematurity, and perinatal mortality