Placenta Praevia

Placenta praevia describes a placenta implanted wholly or partly in the lower uterine segment, classified on ultrasound by the relationship of the placental edge to the internal cervical os: placenta praevia (previously "major," covering the os), and low-lying placenta (placental edge within 20 mm of the internal os but not covering it, previously "minor").

Risk factors include previous cesarean section (risk rises with each prior section, and combined with an anterior low-lying placenta raises specific concern for placenta accreta spectrum), previous placenta praevia, multiparity, multiple pregnancy, and assisted conception.

Classically presents as painless vaginal bleeding, often bright red, in the second half of pregnancy, with a soft, non-tender uterus and, frequently, an abnormal fetal lie or presentation (breech, transverse) since the low placenta prevents the fetal head from engaging.

Bleeding can range from spotting to severe hemorrhage.

• Most cases are identified incidentally on the routine 18 to 20 week anomaly scan; a placenta appearing low at this stage is rescanned at around 32 weeks (placental migration with uterine growth resolves the majority)
• Digital vaginal examination is contraindicated until placenta praevia has been excluded, given the risk of provoking catastrophic hemorrhage
• Transvaginal ultrasound (safe and more accurate than transabdominal) confirms placental position and the precise relationship to the os
• MRI is used in selected cases where placenta accreta spectrum is suspected (particularly with an anterior low placenta over a previous cesarean scar)

• Asymptomatic placenta praevia confirmed after 32 weeks: plan cesarean delivery, typically around 36 to 37 weeks for placenta praevia covering the os, with timing individualized for a low-lying placenta not covering the os (vaginal delivery may be attempted if the edge is more than 20 mm from the os)
• Acute bleeding: manage as major obstetric hemorrhage (see Postpartum Haemorrhage entry for the resuscitation principles, applied equally here): call for senior help, ABC assessment, two large-bore IV access, crossmatch, and admission for observation even if bleeding settles, given the high recurrence risk
• Anti-D prophylaxis for any Rh negative woman with bleeding
• Corticosteroids (betamethasone 12 mg IM x2, 24 hours apart) if preterm delivery is anticipated
• Avoid intercourse and vigorous activity once diagnosed
• Placenta accreta spectrum (abnormally invasive placenta, suspected on imaging with a history of previous cesarean and anterior low placenta) requires delivery planning at a specialist center with interventional radiology, blood bank, and surgical expertise available, given the very high hemorrhage risk; cesarean hysterectomy is often required

Referral: obstetric-led care for all confirmed cases; specialist placenta accreta spectrum service where suspected.