Placenta Accreta Spectrum
Placenta accreta spectrum describes a group of conditions in which the placenta grows too deeply into the wall of the uterus and fails to separate normally after birth. Instead of detaching cleanly, it remains firmly attached, and attempts to remove it cause severe bleeding. The condition has become considerably more common over recent decades, almost entirely because of rising caesarean section rates—the placenta implants over a previous scar, where the normal barrier between the uterine lining and muscle is absent. Its significance lies in planning: when the diagnosis is made before delivery and birth takes place at a specialist centre with a prepared team, outcomes are substantially better than when it is discovered unexpectedly during labour.
What is it?
In a normal pregnancy, the placenta attaches to the lining of the uterus but does not penetrate into the muscle beneath it. A specialised layer of the lining, sometimes called the decidua basalis, forms a boundary that limits how deeply placental tissue invades. After the baby is born, the uterus contracts, this boundary layer shears, and the placenta separates cleanly and is delivered.
Placenta accreta spectrum occurs when that boundary layer is absent or deficient, usually because the uterine lining has been damaged. Placental tissue grows directly onto or into the muscle, and because there is no natural plane of separation, the placenta does not detach after birth. The uterus cannot contract down properly around the retained tissue, and the blood vessels supplying what is by then a large and highly vascular organ remain open. The result is torrential bleeding—this condition is among the leading causes of severe obstetric haemorrhage and of emergency hysterectomy at delivery.
Three degrees are traditionally distinguished by how deeply the placenta has invaded.
Placenta accreta, the mildest and most common, describes placental tissue attached directly to the surface of the muscle without penetrating it.
Placenta increta describes tissue invading into the muscle itself.
Placenta percreta describes tissue penetrating through the full thickness of the uterine wall, potentially reaching the bladder, bowel, or other pelvic structures. This is the most severe and the most surgically demanding.
These are now generally described together as a spectrum rather than separate conditions, since the distinction is often not clear-cut before or even during surgery, and management follows similar principles across the range.
The cause is damage to the uterine lining, and the overwhelming contributor is previous caesarean section. A caesarean scar heals with fibrous tissue that lacks the normal protective lining layer. If a subsequent placenta implants over that scar, it can invade directly into the muscle. This mechanism explains why the condition has risen so markedly—caesarean rates have increased substantially worldwide over several decades, and placenta accreta spectrum has risen alongside them.
The interaction between previous caesarean and placenta praevia—a placenta lying low in the uterus and covering or near the cervix—is the critical combination. The lower part of the uterus is precisely where caesarean incisions are made, so a low-lying placenta in a woman with a previous caesarean is implanting over scar tissue by definition. The risk escalates sharply with each additional caesarean, and a woman with placenta praevia and three or more previous caesareans faces a very high probability of accreta spectrum. This combination is the single most important thing for clinicians to recognise and for women to understand.
Other causes of lining damage contribute. Previous uterine surgery including fibroid removal, repeated surgical management of miscarriage or termination, previous manual removal of a retained placenta, uterine artery embolisation, endometrial ablation, and Asherman syndrome—scarring within the uterine cavity—all increase risk. Advanced maternal age and pregnancy conceived through assisted reproduction are associated, and a scar pregnancy, in which an early pregnancy implants directly into a caesarean scar, is now recognised as being on the same continuum.
The defining feature of this condition clinically is that it usually produces no symptoms at all during pregnancy. A woman typically feels entirely well, and the pregnancy proceeds normally until delivery. This is why detection depends on imaging rather than on recognising symptoms, and why the risk factors matter so much in deciding who should be scanned carefully.
Where bleeding does occur in the second or third trimester, it usually reflects an accompanying placenta praevia rather than the invasion itself, and is characteristically painless. Any bleeding in later pregnancy requires assessment.
Rarely, where the placenta has grown through into the bladder, blood may appear in the urine.
The scenario the condition is best known for is the one that advance diagnosis exists to prevent: an unexpected finding at delivery, when the placenta does not separate, attempts to remove it provoke massive bleeding, and a team that has not prepared for this situation must manage a life-threatening haemorrhage. The difference in outcomes between planned and unplanned management is substantial, and it is the central argument for antenatal detection.
Diagnosis is made by imaging, and the key is knowing who to look at carefully.
The trigger for targeted assessment is straightforward: any woman with a previous caesarean section and a placenta lying low or anteriorly over the scar should have the placental attachment assessed specifically. This combination should prompt detailed scanning rather than routine reporting.
Ultrasound is the primary test and, performed by an experienced operator who is specifically looking, detects the majority of cases. The recognised features include loss of the normal thin clear zone between the placenta and the uterine muscle; multiple irregular vascular lacunae within the placenta, producing a moth-eaten or Swiss-cheese appearance; thinning or loss of the bright line representing the interface between the uterus and the bladder; bulging of placental tissue beyond the expected uterine contour; and abnormal, turbulent, bridging blood flow on Doppler crossing from the placenta into surrounding tissue.
The dependence on operator awareness is worth emphasising. Detection rates are considerably higher where the sonographer knows the woman has had a previous caesarean and is examining the placental attachment deliberately. A substantial proportion of missed cases reflect the question not being asked rather than the features being invisible.
MRI is complementary rather than superior. It is particularly useful where the placenta lies posteriorly and is harder to assess by ultrasound, where ultrasound findings are equivocal, and for defining the extent of invasion into the bladder or beyond when planning surgery. It does not replace ultrasound as the first-line test.
The diagnosis is confirmed at delivery by the clinical findings and by examination of the uterus where it is removed.
Important to Know
The central message is that this condition is managed far better when it is known about in advance. Planned delivery at a specialist centre with an assembled multidisciplinary team substantially reduces blood loss, the need for massive transfusion, intensive care admission, and serious complications compared with unexpected intraoperative discovery. Care should be transferred to a centre with specific experience once the diagnosis is suspected.
The team assembled for a planned delivery typically includes senior obstetricians experienced in this surgery, obstetric anaesthetists, urologists, interventional radiologists, haematology and blood bank support with blood products available in the room, and neonatal staff. This is not excessive preparation—it reflects the realistic possibility of major haemorrhage and the value of having every element in place before it occurs rather than summoning it afterwards.
Timing of delivery balances the risk of unscheduled bleeding against prematurity, and is typically planned between 34 and 37 weeks, earlier where there has been bleeding or where invasion is extensive. Antenatal corticosteroids are given to mature the baby’s lungs. Iron deficiency is corrected during pregnancy so that a woman enters surgery with the best possible reserve.
Caesarean hysterectomy is the standard approach for significant invasion. The baby is delivered through an incision placed away from the placenta, the cord is clamped, and the placenta is deliberately left attached while the uterus is removed with it in place. Attempting to separate the placenta is what causes the catastrophic bleeding, so leaving it undisturbed is the key manoeuvre. This means losing the uterus and therefore future fertility, which is a profound loss for many women and should be discussed thoroughly and early in pregnancy rather than at the point of surgery.
Uterus-preserving approaches exist and are used in selected cases at experienced centres. These include leaving the placenta in place entirely and allowing it to resorb over subsequent months with close monitoring, or resecting the affected portion of the uterine wall and repairing it. Both can preserve fertility, and successful subsequent pregnancies have been reported. Both also carry risks—delayed haemorrhage, infection, and the possibility of requiring hysterectomy later—and they are not suitable for all cases, particularly extensive percreta. Where preserving fertility matters greatly, it is reasonable to ask whether a centre offers these approaches and whether the specific anatomy would permit them.
Various adjuncts are used to reduce bleeding, including balloon catheters placed in the pelvic arteries before surgery and ready to be inflated, and interventional radiology embolisation. Evidence for their benefit is mixed and practice varies between centres.
Blood transfusion is frequently required, and in some cases substantially. Cell salvage, which collects and returns a woman’s own blood during surgery, is used in many centres.
Bladder involvement is common in percreta and may require repair or, occasionally, partial removal, which is why urological input is part of the planned team.
The psychological impact deserves genuine attention. Being told during pregnancy that delivery will likely involve hysterectomy, major blood loss, and a specialist team is frightening, and it arrives at a time that should be one of anticipation. For women who intended to have more children, the loss of fertility is a bereavement that coexists with a new baby. Support during pregnancy and afterwards, including from psychological services and from others who have been through it, is valuable and should be offered rather than requested.
Recovery after caesarean hysterectomy is longer than after a straightforward caesarean, and women should expect this. Where the placenta is left in place, follow-up is prolonged, with monitoring for bleeding and infection over months.
Prevention is relevant at a population level and worth stating plainly. The condition has risen because caesarean rates have risen. Avoiding a first caesarean where it is not clinically necessary is the most effective preventive measure available, and supporting vaginal birth after caesarean where appropriate reduces cumulative exposure. This is not an argument against caesarean where it is indicated—it is a reason that decisions about a first caesarean carry implications for future pregnancies that are worth understanding at the time.
For a woman who has had a previous caesarean and is planning another pregnancy, the practical implication is that early assessment of placental position in that pregnancy matters, and she should ensure her obstetric team knows her surgical history in detail.
Care is coordinated by specialist maternal-fetal medicine and obstetric teams, with urology, interventional radiology, anaesthesia, haematology, and neonatal services. Imaging findings are interpreted alongside surgical history, placental position, and clinical circumstances rather than in isolation.
Patient education plays an important role. Understanding that the condition usually causes no symptoms, that detection depends on targeted imaging in women with risk factors, that planned delivery at a specialist centre markedly improves outcomes, that hysterectomy is often necessary and why, and that uterus-preserving options exist but are selective all contribute to appropriate care and to informed participation in decisions.
Red flag symptoms include any vaginal bleeding in the second or third trimester, particularly if painless, which requires immediate assessment; heavy bleeding with dizziness, faintness, breathlessness, or a racing heart, which requires emergency care; regular painful contractions or signs of labour before the planned delivery date in a woman with a known diagnosis, which should prompt immediate attendance at the planned delivery hospital rather than a local unit; abdominal pain; reduced fetal movements; and blood in the urine. Women with a known diagnosis are generally given a specific plan for where to go and whom to contact, and should follow it rather than attending the nearest unit.