What is placenta accreta spectrum (PAS)? Let’s look at symptoms, diagnosis, and treatments.

What is placenta accreta spectrum (PAS)? Let’s look at symptoms, diagnosis, and treatments.

by | Jan 19, 2024 | QUFW News

Placenta accreta spectrum is a potentially life-threatening obstetric condition where the placenta grows too deeply into the wall of the uterus. This abnormality may prevent the placenta from detaching after childbirth, causing significant maternal bleeding and other complications for both the mother and her baby. 

Placenta accreta was traditionally classified into different types based on the severity and depth of placental invasion (accreta, increta and percreta). Often, however, the only way to determine this is after delivery. As a result, all disorders of abnormal placentation are now referred to collectively as placenta accreta spectrum (or PAS) 

PAS is often diagnosed during pregnancy through ultrasound, although it may sometimes only be detected during delivery. It requires advanced diagnosis and careful management by a multidisciplinary medical team.

This fact sheet answers these questions:

What is placenta accreta spectrum (PAS)? 

‘Placenta accreta spectrum disorder’ (PAS) is a pregnancy-related condition where the placental villi abnormally invade the myometrium during the first trimester of pregnancy. 

Normally, the placenta separates from the wall of the uterus after childbirth. However, in cases of PAS, this detachment does not occur as it should.


Animated portrayal of placenta accreta spectrum (PAS) – John Hopkins Medicine ​

 

A study covering Australia and Aotearoa New Zealand found that the occurrence of PAS is around 44.2 in every 100,000 women (or 1 in 2000) who are giving birth, although there are some risk factors which make PAS much more likely in some patients. 

What are the different types of PAS? 

PAS is classified into three types based on the depth of placental invasion into the wall of the uterus. This differentiation can only definitively be made after delivery, on histopathological  assessment. 

Placenta accreta: This is the most common. In placenta accreta, the placenta attaches itself deeply into the uterine wall but does not penetrate the muscle of the uterus. This can lead to difficulties in placental detachment during childbirth.

Placenta increta: In this form, the condition is more severe. The placenta invades into the muscles of the uterus, embedding itself more deeply than in placenta accreta. This increases the risk of severe bleeding during delivery and can pose more significant health risks.

Placenta percreta: This is the most severe form of the condition. In placenta percreta, the placenta penetrates through the entire uterine wall and can attach to other organs, like the bladder or intestines. This type poses the highest risk of severe complications, including severe bleeding, and may require extensive surgical intervention, and earlier delivery.

The severity and extent of PAS significantly impacts the approach to delivery and the type of medical intervention required. Early and accurate diagnosis is essential for managing these risks and planning a safe delivery strategy.

Image source – Semantic Scholar

How is PAS diagnosed? 

PAS is typically diagnosed through a combination of imaging tests and clinical assessment. The most common diagnostic methods include:

  • Ultrasound: Ultrasound is the first line imaging modality for diagnosing PAS. A detailed ultrasound can show abnormal placental attachment and blood flow. Doppler ultrasound, which visualises blood flow, can be particularly useful in identifying unusual blood vessels that may suggest the presence of PAS.
  • Magnetic Resonance Imaging (MRI): If ultrasound results are inconclusive or if a more detailed view is needed, an MRI can be used. MRI has similar sensitivity in detection of PAS as ultrasound, but can be useful when ultrasound is limited (e.g. in higher BMI, or when the placenta is posterior) . 
  • Clinical History and Risk Factor Assessment: A thorough assessment of the patient’s medical history, including previous caesarean sections, uterine surgeries, and other risk factors, is important. This information (also known as the pre-test probability, or likelihood), combined with imaging results, helps in making a diagnosis.

Early diagnosis is crucial for planning appropriate medical care to manage the condition and reduce the risk of complications during delivery. Women with known risk factors for PAS are typically monitored more closely with these diagnostic tools, and an accurate antenatal diagnosis of PAS impacts delivery planning.

Placenta praevia completely covering the internal cervical os with placenta accreta

What risk factors may contribute to PAS?

There is some thought that PAS may be a later gestation manifestation of caesarean-scar pregnancy, or a pregnancy which implants within the prior caesarean scar. Rather than being an abnormality innate to the placenta itself, the abnormal invasion and development of the placenta may relate to the location in which it implanted, and the nature of the scar tissue. 

PAS is known to be associated with several risk factors, each contributing to the increased likelihood of its occurrence:

  • Previous Caesarean Delivery: A history of caesarean delivery is a major risk factor for PAS. With each subsequent caesarean, the risk of PAS increases. This is because the surgical scar in the uterus can be a site where the placenta abnormally adheres in future pregnancies.
  • Low-Lying Placenta or Placenta Previa: When the placenta is positioned low in the uterus or covers the cervix (placenta previa), the risk of PAS is higher. This abnormal positioning can lead to a deeper implantation of the placenta into the uterine wall. However, placenta accreta spectrum is still relatively rare in placenta praevia, unless a patient has also had prior caeasarean sections.
  • Advanced Maternal Age: Women who are older, typically over the age of 35, have a higher risk of developing PAS. 
  • Multiparity: Having multiple pregnancies increases the risk of PAS. 
  • Previous Uterine Surgery Including Myomectomy, D&C, Cornual Resection, or Endometritis: Any previous uterine surgery, such as the removal of fibroids (myomectomy), dilation and curettage (D&C), cornual resection, or a history of endometritis (inflammation of the uterine lining), can leave scars or changes in the uterus. These alterations can create areas where the placenta can attach too deeply, increasing the risk of PAS.

Transvaginal ultrasound image demonstrating placenta praevia with suggestion of placenta increta. Bladder wall appears intact.

What are the symptoms of PAS? 

PAS often does not present with multiple distinct symptoms, especially early in pregnancy, which is why it is frequently diagnosed through routine imaging in at-risk patients. However, there are a few signs and symptoms that may suggest the presence of PAS, particularly as a pregnancy progresses:

  • Vaginal Bleeding: This is the most common symptom and usually occurs in the second or third trimester of pregnancy. The bleeding can range from light to heavy.
  • Fetal malpresentation: The baby may persistently remain in a non-cephalic (breech or transverse) position because the abnormally implanted placenta (when covering the cervix) prevents the baby descending in the pelvis.

It’s important to note that many women with PAS do not experience any noticeable symptoms, and the condition is often detected during routine prenatal imaging in women who have risk factors for the condition.

What are the risks associated with PAS? 

PAS carries several significant risks and potential complications, mainly due to the abnormal attachment of the placenta to the uterine wall. These risks can impact both the mother and the baby:

  • Severe Maternal Haemorrhage: This is the most significant risk. The deep attachment of the placenta can lead to severe bleeding during attempts to remove the placenta after childbirth. This can result in a life-threatening situation requiring immediate medical intervention.
  • Need for Hysterectomy: Most commonly, a hysterectomy (surgical removal of the uterus) is necessary following delivery, as the placenta will not detach from the uterine wall. 
  • Preterm Birth: PAS can increase the risk of preterm labour and delivery, which carries risks for the baby, including respiratory distress syndrome, developmental delays, and other prematurity-related complications. This can be either iatrogenic (planned preterm birth to reduce potential complications), or spontaneous, in which case delivery may be indicated in the scenario of a large bleed. 
  • Blood Transfusions and Surgery: Due to the risk of severe bleeding, blood transfusions and multiple surgical interventions may be required.
  • Damage to Other Organs: In severe cases, particularly in placenta percreta, the placenta can invade other organs like the bladder or intestines, leading to additional complications and the need for more complex surgery.
  • Mental Health Impact: The diagnosis and management of PAS, including the potential loss of fertility and the stress of a high-risk pregnancy, can have significant emotional and psychological impacts.

For the baby, the risks are mainly associated with premature birth and the potential complications that can arise from being born early. There is also a risk of fetal growth restriction associated with placenta accreta spectrum, and this is why serial ultrasound is recommended to monitor fetal growth.

Transvaginal ultrasound image demonstrating colour flow in placenta accreta.

How is PAS treated? 

The treatment of PAS primarily focuses on managing the risks during childbirth and addressing potential complications, especially severe bleeding. The treatment plan depends on the severity of the condition, the health of the mother and baby, and the stage of pregnancy. 

Common treatment strategies include:

  • Planned Caesarean Hysterectomy: Women with PAS will be recommended to deliver by planned caesarean section. This is often planned around 34-36 weeks of gestation, to minimise the risk of spontaneous labour, which could lead to uncontrollable bleeding. In most cases, with the exception of small focal placenta accreta spectrum, a hysterectomy (removal of the uterus) is planned as a part of the same operation and performed immediately after the delivery of the baby. This is often necessary to control bleeding and can be a life-saving measure.
  • Blood Transfusions: Due to the high risk of severe bleeding, arrangements for blood transfusions are usually made in advance of the delivery.
  • Medication Management: Medications may be used to control bleeding, manage pain, and prevent infection during and after the delivery.
  • Follow-up Care: Postpartum care is crucial, especially if a hysterectomy is performed. Monitoring for complications like infection, bleeding, or psychological impact is important.

Each case of PAS is unique, and the treatment plan is tailored to the individual’s specific situation. Early diagnosis and careful planning are key to the successful management of PAS.

Will I need a Caesarean section if I have PAS? 

If you have PAS, a caesarean section will be recommended for delivery. This is because the abnormal attachment of the placenta to the uterine wall in PAS makes it impossible for the placenta to separate naturally and safely during vaginal delivery. Attempting a vaginal birth in such cases could lead to severe, life-threatening bleeding.

In many cases a hysterectomy (surgical removal of the uterus) will be planned immediately after the delivery of the baby during the same surgical procedure. This is done to control bleeding and is often a necessary step in managing the condition.

It’s important to discuss your specific case with your healthcare provider, who can advise on the best approach for delivery based on the extent of the PAS and your overall health. They will also consider other factors, such as whether the PAS is considered likely to be focal (a small area which may be able to be resected) or more diffuse.

Can PAS be prevented? 

Preventing PAS can be challenging because the exact cause of the condition is not entirely understood. However, there are certain measures and considerations that can help reduce the risk:

  • Limiting Caesarean Deliveries: Since a major risk factor for PAS is previous caesarean deliveries (Caesarean sections), limiting the number of Caesarean sections when medically possible can reduce the risk. 
  • Careful Management of Uterine Surgeries: Procedures that involve the uterus, such as myomectomy (removal of uterine fibroids), can increase the risk of PAS in future pregnancies. This is more the case when fibroidectomy is cavity breaching.
  • Early Prenatal Care: Early and regular prenatal care is important, especially for women with risk factors for PAS. Early detection of potential issues allows for better planning and management.
  • Family Planning and Birth Spacing: Thoughtful family planning and spacing between pregnancies can help minimise the cumulative risk associated with multiple pregnancies and Caesarean sections.

Can I still have a vaginal birth with PAS?

Most cases of placenta accreta spectrum also occur in the setting of a low lying or praevia placenta, which will often prevent descent of the baby into the birth canal to allow a vaginal delivery. 

Additionally, when the placenta is abnormally adherent, even if the baby can be delivered vaginally, the PAS will prevent placental separation warranting operative management following delivery to manage bleeding and retained placenta. 

Further Reading

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