Placenta Accreta Spectrum

Placenta accreta spectrum is a group of conditions characterised by abnormal, pathological adherence of the placenta to the uterine wall. It results from a defect in the normal decidual layer, typically associated with a previous uterine scar, most commonly previous caesarean delivery. It is an important and increasingly prevalent cause of massive obstetric haemorrhage, given the rising rate of caesarean delivery in many populations.

It is classified by depth of abnormal placental invasion:

• Placenta accreta: invasion confined to the superficial myometrium.
• Placenta increta: invasion into the deeper myometrium.
• Placenta percreta: invasion through the full thickness of the myometrium and, in the most severe cases, into adjacent pelvic structures including the bladder.

Antenatal diagnosis and structured, multidisciplinary delivery planning are essential. Planned, appropriately resourced management is directly linked to improved maternal outcome, compared with unanticipated, emergency presentation.

Pathophysiology and risk factors

• Normal placental attachment depends on an intact decidual layer separating the placental trophoblast from the underlying myometrium.
• In placenta accreta spectrum this decidual layer is defective, most commonly at the site of a previous uterine scar. This allows abnormal, direct trophoblastic invasion into and, in more severe cases, through the myometrium.
• The single most important risk factor is placenta praevia together with a previous caesarean delivery. Risk increases substantially with the number of previous caesarean deliveries.
• This reflects the cumulative relationship between the extent of previous uterine scarring and the likelihood of abnormal placental attachment at the scar site, where the placenta implants over or adjacent to that location in a subsequent pregnancy.
• Other recognised risk factors include previous uterine surgery of any type (including myomectomy or dilation and curettage), advanced maternal age, and multiparity.
• The combination of placenta praevia and previous caesarean delivery is by far the most significant risk factor combination. It warrants proactive antenatal consideration and screening.

• The condition is ideally identified through structured antenatal screening in an asymptomatic woman with relevant risk factors, rather than through symptomatic presentation. Antenatal diagnosis and planned, structured delivery management bring substantial, direct benefit compared with unanticipated presentation at delivery.
• Where not identified antenatally, it characteristically presents dramatically at the time of attempted placental delivery. The placenta fails to separate normally, and massive, life-threatening haemorrhage follows attempts to manually remove the abnormally, deeply adherent placenta. This is an immediate obstetric emergency.
• Placenta percreta, the most severe pattern with invasion through the full uterine wall thickness, may present antenatally with haematuria (where there is bladder invasion) or, rarely, with uterine rupture. The great majority of cases remain clinically silent until delivery unless identified through proactive antenatal screening.
• The key clinical context is a history of placenta praevia together with one or more previous caesarean deliveries. This warrants proactive consideration of the diagnosis and targeted antenatal screening.

• Antenatal ultrasound, performed by a practitioner with specific expertise in this assessment, is the primary screening and diagnostic investigation. It supports diagnosis and gives some indication of the likely depth of invasion.
• Characteristic sonographic features: loss of the normal, clear hypoechoic retroplacental myometrial zone; placental lacunae (irregular, vascular spaces within the placenta producing a characteristic "Swiss cheese" appearance); and abnormal vascularity at the uterovesical interface on colour Doppler assessment.
• Magnetic resonance imaging is a valuable, complementary investigation. It gives more detailed assessment of the extent and depth of placental invasion, and of any suspected invasion into adjacent pelvic structures. It is especially relevant for posteriorly located placentas, where ultrasound assessment can be technically more challenging. It directly informs surgical planning.
• Structured, proactive antenatal screening for placenta accreta spectrum is an essential, standard component of antenatal care in this higher-risk population. It uses targeted ultrasound assessment in any woman with placenta praevia and previous caesarean delivery. Antenatal diagnosis enables appropriately planned delivery management.

• Once antenatal diagnosis is established, the central approach is structured multidisciplinary delivery planning. It involves senior obstetric, anaesthetic, urological (given the potential for bladder involvement in more severe cases), interventional radiology, and neonatal expertise.
• Deliver at an appropriately resourced specialist centre with immediate access to substantial blood product support and critical care facilities.
• Planned, elective caesarean delivery is the standard approach. It is generally performed somewhat earlier than standard term delivery timing, balancing fetal maturity against the substantial risk of spontaneous labour or antepartum haemorrhage precipitating an unplanned, less well-resourced emergency delivery. Timing is individualised to the clinical circumstances.
• Planned caesarean hysterectomy is the standard, definitive management for the great majority of confirmed, significant cases. It is performed by an experienced surgical team at the time of delivery, without any attempt at placental separation, given the catastrophic haemorrhage risk of attempted removal of an abnormally, deeply adherent placenta. The placenta is left in situ within the uterus at hysterectomy.
• Conservative, uterus-preserving management is an alternative, more complex approach in carefully selected cases. It is considered particularly where future fertility preservation is a significant priority for the woman.
• Options include leaving the placenta in situ with subsequent expectant management or, in carefully selected cases, adjunctive treatment including methotrexate or uterine artery embolisation.
• This approach carries a recognised, substantial risk of complications including delayed haemorrhage and infection. Counsel the woman carefully and individually on the relative risks and benefits compared with the more standard, definitive hysterectomy approach.
• Prepare proactively for massive haemorrhage. Cross-match substantial quantities of blood products in advance of planned delivery. Use intraoperative cell salvage where available. Ensure immediate availability of interventional radiology support for balloon occlusion or embolisation techniques, where relevant to institutional protocol and case complexity.
• Provide psychological support addressing the impact of the pregnancy complication and, where hysterectomy has been required, its implications for future fertility. Provide postoperative care and structured follow-up.