Placenta accreta is most commonly diagnosed during the second trimester, typically at the 18- to 22-week anatomy scan, though it can be detected or confirmed later in pregnancy and sometimes isn’t discovered until delivery itself. The timing depends on risk factors, imaging quality, and how deeply the placenta has invaded the uterine wall.
First Trimester Screening Is Limited
In the first trimester, there are no reliable imaging signs specific enough to diagnose placenta accreta spectrum (PAS). Some blood markers show subtle changes early on. Levels of PAPP-A (a protein measured in standard first-trimester screening) tend to be elevated in women who later develop PAS, and cell-free fetal DNA circulating in the mother’s blood may be unusually high. These findings are still considered experimental and aren’t used clinically to make or rule out a diagnosis. What first-trimester care does accomplish is identifying risk factors, particularly a history of cesarean delivery combined with a low-lying placenta, that flag the need for closer imaging later.
The 18- to 22-Week Anatomy Scan
The most common point of initial detection is the routine mid-pregnancy ultrasound, performed between 18 and 22 weeks. At this stage, the placenta is large enough and the uterine wall thin enough for several telltale signs to become visible. The hallmark finding is abnormal lacunae: large, irregular, fluid-filled spaces scattered through the placenta that give it a “moth-eaten” appearance on the screen. These lacunae form where high-pressure blood vessels from deep in the uterine wall have disrupted normal placental tissue.
Other signs radiologists look for include myometrial thinning, where the muscle layer behind the placenta measures less than 1 millimeter or becomes completely undetectable. This happens because the placental cells have migrated through the scarred or weakened muscle, leaving little tissue behind. When the placenta sits low on the front wall of the uterus (anterior placenta previa), a cluster of newly formed blood vessels may appear between the uterus and the bladder. On ultrasound, the walls of these vessels create a characteristic pattern of parallel bright lines, sometimes called the “equals sign,” or a scalloped look along the bladder surface.
At this gestational age, however, ultrasound picks up only about half of true cases. One study found sensitivity of just 51.6% at 18 to 22 weeks, with a specificity of 79.2%. That means a normal-looking scan in the second trimester doesn’t rule it out, which is why women with strong risk factors get repeat imaging later.
Third Trimester Imaging Is More Accurate
A follow-up ultrasound between 32 and 34 weeks catches significantly more cases. Sensitivity jumps to roughly 90% at this stage, largely because the signs of abnormal invasion become more pronounced as the placenta grows and blood flow increases. The tradeoff is slightly lower specificity (about 61%), meaning more false positives, but the high detection rate makes this window critical for surgical planning.
For women whose ultrasound results are unclear, or who have high clinical risk but ambiguous imaging, MRI can be ordered as a secondary tool. MRI is particularly useful for evaluating how far the placenta has grown into or through the uterine wall, which helps distinguish the milder form (accreta, where villi attach to the muscle surface) from the more dangerous form (percreta, where villi penetrate through the uterus and potentially into the bladder or other organs). Signs that point toward percreta on MRI include dark bands running through the placenta, a mixed-texture placental appearance, and visible disruption of the bladder wall.
That said, MRI doesn’t always improve the picture. Research shows it correctly changes the diagnosis in about 19% of cases and confirms the ultrasound findings in 44%, but it incorrectly changes the diagnosis in 17%. Because of its high cost and these limitations, it isn’t recommended as a routine addition to ultrasound for every suspected case.
What Ultrasound Signs Suggest Deeper Invasion
Not all cases of PAS carry the same risk, and imaging can help estimate severity before delivery. The two ultrasound features most strongly associated with percreta (the deepest invasion, where placental tissue reaches the outer surface of the uterus) are increased blood vessel activity at the boundary between the uterus and the bladder, and a high number of lacunae that lack the normal bright border around them. The bladder vascularity finding carried nearly eight times the odds of percreta compared to a shallower invasion. On MRI, bladder wall interruption was the single strongest predictor, associated with roughly 16 times the odds of percreta.
Final confirmation of how deeply the placenta invaded only comes after delivery, either through direct visual observation during surgery or through microscopic examination of the uterus if a hysterectomy is performed.
Diagnosis During Delivery
Some cases of placenta accreta are never caught on prenatal imaging and are only recognized at the time of birth. This is more likely in women without obvious risk factors or those who had limited prenatal care. The classic scenario: after the baby is born, the placenta doesn’t separate on its own within 30 minutes. When the delivering team attempts to remove it manually, they can’t find a natural plane of separation between the placenta and the uterine wall. Pulling on the placenta triggers heavy, sometimes life-threatening bleeding.
In some cases, bleeding before the placenta is delivered is minimal or even absent, which can delay recognition. The inability to separate the placenta combined with hemorrhage during attempted removal are the defining clinical signs.
Why Early Detection Matters
The reason timing matters so much is that a prenatal diagnosis allows a planned cesarean delivery, typically around 34 to 36 weeks, at a hospital with the right surgical team, blood bank support, and possibly a urologist on standby for cases involving the bladder. Women diagnosed before delivery have significantly better outcomes than those caught off guard in the delivery room. The rate of PAS has been rising alongside the increase in cesarean deliveries. In women with multiple prior cesareans, the incidence climbs steeply, reaching 2.8% in those with the highest number of prior cesarean births, according to a large study covering 2011 through 2024.
If you have a prior cesarean scar and a placenta that sits low on the front wall of the uterus, targeted imaging at the anatomy scan and again in the early third trimester gives the best chance of catching the condition in time to plan a safe delivery.