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Placenta previa and placenta accreta spectrum

  • Lesion

If the placenta lies over the cervix or grows into the uterine wall where there is no lining, birth can cause catastrophic bleeding.

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What it is

Placenta previa is implantation in the lower uterine segment, partly or completely over the cervical os; it causes painless bleeding in the third trimester and requires caesarean delivery when the os is covered. In the placenta accreta spectrum, the decidua is absent or defective, usually over a caesarean scar, so villi attach directly to the myometrium (accreta), invade it (increta) or pass through it to the serosa and nearby organs (percreta). The placenta cannot separate after birth, causing massive postpartum haemorrhage.

Structure

  • Hysterectomy specimen: villi in direct contact with smooth muscle without intervening decidua, sometimes reaching the serosa.
  • The implantation site often lies over a thinned caesarean scar.
  • Abruption: a retroplacental clot compressing the villi, with infarction of the overlying tissue if long-standing.

Step by step

  1. A previous caesarean, curettage or other surgery damages the endometrium and leaves a scar without decidua.
  2. The blastocyst implants over the scar, often in the lower segment (previa).
  3. With no decidua to limit them, villi and extravillous trophoblast attach to and invade the myometrium.
  4. After delivery the placenta cannot separate, and the uterus cannot contract to close the bleeding vessels.

Clinical notes

  • Rising caesarean rates have made the accreta spectrum more common; ultrasound and MRI are used to plan delivery in high-risk women.
  • Planned delivery in a centre with blood bank and surgical support reduces maternal deaths.
  • Digital vaginal examination is avoided in known previa because it can provoke bleeding.

Test yourself

Think of the answer, then tap the question to see it.

What is the underlying defect in placenta accreta?

Absence or deficiency of the decidua basalis, so villi attach directly to the myometrium.

What is the main risk factor for the accreta spectrum?

Previous caesarean delivery, especially combined with placenta previa over the scar.

How does placenta previa usually present?

With painless bright red vaginal bleeding in the third trimester.

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Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: Placenta Previa (StatPearls (NCBI Bookshelf), 2026); Placental Abruption (StatPearls (NCBI Bookshelf), 2026).

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