Placenta Previa & Accreta
- Previa is a location problem: the placenta sits too low and covers (or nearly covers) the internal cervical os — the baby's exit.
- Accreta spectrum is an attachment problem: the placenta grows in too deep and refuses to let go after delivery.
- The two travel together: a previa over an old C-section scar is the classic setup for accreta.
- Ultrasound is the front-line tool; MRI is the tiebreaker when the picture is murky or the placenta is posterior.
- Both can cause catastrophic bleeding, so the radiologist's job is basically to give the OB a heads-up before the delivery, not after.
Think of the placenta as a houseguest. A good houseguest sets up in a reasonable spot and leaves politely when it's time to go. Previa and accreta are the two ways that guest goes wrong: previa parks right in front of the only door, and accreta nails its furniture to the floor. Both are bad news at checkout — which, in obstetrics, is a delivery — and both are things we'd much rather diagnose with an ultrasound probe than discover in a panic in the operating room.
Previa: parked in front of the door
The cervix has an internal opening called the internal os — the top of the birth canal, the door the baby is supposed to leave through. Placenta previa simply means the placenta is sitting over that door.
We grade it by how the placenta relates to the os. If any placental tissue covers the internal os, that's a placenta previa. If the edge stops short of the os but lands within about 2 cm of it, we call that a low-lying placenta rather than a true previa. (You may still hear older terms like complete, partial, and marginal previa, but the current convention has mostly collapsed them into these two buckets.) The exact edge-to-os distance matters because it changes whether a vaginal delivery is even on the table.
Early in pregnancy, a low-looking placenta is incredibly common and usually nothing to panic about. As the uterus stretches upward through the second and third trimesters, most of these "migrate" away from the cervix. So a previa seen at 20 weeks often resolves on its own — you confirm it later, not condemn it early.
The measurement itself is a classic trap. A full bladder squashes the lower uterus and can fake a previa where none exists; contractions can do the same. The honest answer is to image with the bladder partly emptied, and when the transabdominal view is ambiguous, go transvaginal — counterintuitively safe here, and far more accurate for measuring that edge-to-os distance.
An overfull maternal bladder is the great previa impostor. It pushes the anterior and posterior lower uterine walls together, making a normal placenta look like it's draped over the os. If a "previa" vanishes after the patient voids, it was never real.
Accreta spectrum: furniture nailed to the floor
Normally the placenta sits on a tidy buffer layer of uterine lining (the decidua) and peels away cleanly after birth. The placenta accreta spectrum (PAS) is what happens when that buffer is missing — usually because a prior C-section scar wrecked it — and the placental tissue burrows into the muscle instead.
There's a depth ladder, and the names just describe how far in it went:
| Term | How deep the placenta invades |
|---|---|
| Accreta | Attaches to the myometrium, but doesn't grow into it |
| Increta | Grows into the muscle wall |
| Percreta | Grows all the way through, sometimes invading the bladder |
The villain in the origin story is almost always a previous uterine scar. A previa lying directly over an old C-section scar is the highest-risk combination there is, and the risk climbs with each additional prior cesarean. So when you see a low anterior placenta in someone with a surgical history, accreta is the question you must ask out loud.
The single most useful pre-imaging fact is the patient's history: prior cesareans plus a current previa is the accreta setup. Read the chart before you read the scan — knowing the risk tells you how hard to hunt for the findings.
What you're hunting for on imaging
On ultrasound, the clues are about a lost boundary. Normally there's a thin dark line — the clear zone — between placenta and bladder. In accreta that line disappears. You also look for placental lacunae (irregular vascular spaces that give the placenta a moth-eaten, "Swiss cheese" look), thinning or loss of the bladder-wall interface, and a chaotic tangle of vessels crossing the boundary on color Doppler.
When ultrasound is equivocal — a posterior placenta, a big patient, or surgical planning that needs a roadmap — we reach for pelvic MRI. On MRI the tells are dark intraplacental bands on T2, focal bulging of the uterine contour, and disorganized vessels. MRI doesn't replace ultrasound; it referees the close calls and maps the extent before surgery.
Why the radiologist's call matters
This is one of those diagnoses where the reading genuinely changes the day. A flagged accreta means a planned, scheduled delivery with the right team, the right room, and blood ready to go — instead of an emergency. Its close cousin, placental abruption, is the acute bleeder; previa and accreta are the ones you catch ahead of time. If you remember one thing: previa is about where the placenta is, accreta is about how stuck it is — and a previa over an old scar is your cue to check for both.