Obstetric Topic 30. Placenta previa. Other placental abnormalities

I. Placenta Previa

Definition and Types

• Placenta previa / placenta praevia: abnormal implantation in the lower uterine segment, covering the internal cervical os.
• Classification:
  1. Type I / low-lying placenta: placental edge in lower segment, usually <2 cm from internal os, but not covering it.
  2. Type II / marginal: placental edge reaches the internal os but does not cover it.
  3. Type III / partial: placenta partially covers the internal os.
  4. Type IV / complete: placenta completely covers the internal os.
• Placental "migration": lower uterine segment growth → many 2nd-trimester low placentas resolve by the 3rd trimester.

Risk Factors and Clinical Features

• Risk factors: previous cesarean section, uterine surgery/D&C, previous placenta previa, multiparity, advanced maternal age, multiple pregnancy.
• Classic presentation: painless bright-red vaginal bleeding after 20 weeks, usually in the 3rd trimester.
• Bleeding may be recurrent; uterus is usually soft and non-tender.
• Malpresentation: breech/transverse lie because placenta occupies the lower segment.
• Associated risks: PPROM, preterm birth, IUGR/FGR, vasa previa, postpartum hemorrhage, placenta accreta spectrum.

Diagnosis and Management

• Diagnosis: transvaginal ultrasound is gold standard and safe; transabdominal ultrasound may screen placental location.
• Digital vaginal examination is contraindicated until placenta previa is excluded by ultrasound; speculum examination may be used if needed.
• Initial bleeding care: assess maternal hemodynamics, IV access, CBC, blood group/crossmatch, coagulation if heavy bleeding, fetal monitoring if viable.
• Expectant management if preterm and bleeding stops, especially <34 weeks:
  • Observation, rest/pelvic rest, monitor bleeding and fetal well-being.
  • Antenatal corticosteroids if preterm delivery is likely.
  • Anti-D immunoglobulin if Rh-negative.
• Delivery: cesarean section for placenta previa, usually planned around 36-37 weeks if stable.
• Active bleeding / instability: resuscitate with IV fluids, blood products as needed and urgent cesarean delivery.
• Low-lying placenta: vaginal birth may be possible when placental edge is sufficiently far from the os and bleeding risk is acceptable; decide by distance, ultrasound, symptoms and local protocol.

II. Placenta Accreta Spectrum

Definition and Types

• Placenta accreta spectrum (PAS): abnormal placental attachment/invasion through defective decidua basalis, often at a uterine scar.
• Mechanism: defective decidua/Nitabuch layer → chorionic villi attach too deeply → placenta does not separate → massive hemorrhage.
• Types:
  1. Placenta accreta: villi attach to myometrium without invasion.
  2. Placenta increta: villi invade into myometrium.
  3. Placenta percreta: villi penetrate through myometrium to serosa or adjacent organs.

Risk Factors, Diagnosis and Management

• Risk factors: previous cesarean section is most important; also uterine surgery/D&C, placenta previa, multiparity and advanced maternal age.
• Clinical clue if undiagnosed: retained placenta or attempted manual separation → profuse life-threatening postpartum hemorrhage.
• Ultrasound/Doppler: loss of clear uterine-placental interface, placental lacunae, myometrial thinning, abnormal vessels.
• MRI: assesses depth of invasion when ultrasound is unclear or deep invasion is suspected.
• Management: planned cesarean delivery at about 34-36 weeks in a specialist center with blood bank and multidisciplinary team.
• Severe/suspected PAS: cesarean hysterectomy is common; placenta is usually left in situ. Do not forcibly remove placenta.

III. Other Placental Abnormalities

Vasa Previa

• Definition: unprotected fetal vessels run in membranes over/near the internal cervical os.
• Risk factors: low-lying placenta/resolved previa, velamentous cord insertion, succenturiate/bilobed placenta, multiple pregnancy.
• Presentation: painless bleeding after rupture of membranes + fetal bradycardia/sinusoidal or non-reassuring FHR; blood is fetal.
• Diagnosis: transvaginal ultrasound with color Doppler.
• Management: planned cesarean before labor/ROM; emergency cesarean for bleeding, ROM or fetal distress.

Placental Abruption

• Definition: premature separation of a normally implanted placenta after 20 weeks.
• Previa vs abruption: previa is typically painless with soft uterus; abruption is painful with tender/hypertonic uterus and fetal distress.
• Details are covered in O32.