Placenta accreta spectrum disorders: Abnormally adherent and invasive placenta

Abnormally adherent and invasive placenta (also known as placenta accreta spectrum, PAS) is caused by abnormal implantation of the placenta. This results in an excessive invasion of placental villi into the myometrium. 
Although PAS is a spectrum disorder, there are three commonly recognised types defined by the depth of penetration of the placental villi into the myometrium¹:  

• Placenta accreta: the villi abnormally adhere to the myometrium but do not invade it. 
• Placenta increta: the villi penetrate deeply into the myometrium.
• Placenta percreta: the villi pass through the myometrium reaching the uterine serosa and sometimes beyond with invasion into adjacent organs such as the bladder. 

Abnormally invasive placenta includes both placenta increta and percreta whereas abnormally adherent placenta is defined by placenta accreta.²
 

Placenta accreta spectrum disorders prevent normal separation of the placenta at delivery, resulting in a high risk of severe post-partum hemorrhage. 
Abnormally invasive placenta (increta and percreta) is a life-threatening condition, and a major cause of caesarean-hysterectomy and maternal morbidity and mortality. 
PAS can be total, partial or focal.   
 

O43.2 Morbidly adherent placenta

The prevalence in the general population of pregnant women is around 1.7 per 10,000 pregnancies.^{3, 4}
The incidence of PAS is 4.1% in women with one prior cesarean delivery and 13.3% in women with two or more previous caesarean deliveries.
The risk of PAS in a subsequent pregnancy is between 22 to 29%.⁴
 

There are few studies on the etiopathogenesis of abnormally adherent and invasive placentation. The current hypothesis is that a defect of the endometrium-myometrium interface (caused by previous caesarean for example) leads to a failure of normal decidualisation in the area of uterine scar, allowing abnormally deep placenta anchoring villi and trophoblast infiltration.⁵ Recent studies show that there are no morphological differences between the normal and PAS placenta.⁶

Prenatal diagnosis of PAS is vital as it facilitates appropriate referral to a team experienced in the complexities such deliveries present and has been shown to reduce maternal morbidity.^{3, 8-10}
The difference between placenta accreta and increta can only be shown by histological findings. A definitive clinical diagnosis can only be made at delivery and should be confirmed by histopathology wherever possible. In the FIGO consensus guidelines, there is a clinical grading system to categorize placental adherence at delivery.⁷ This clinical grading system is below: 

Prenatal ultrasound diagnosis is based on a number of different signs. Standardised descriptions of these signs were proposed by the European Working Group on Abnormally Invasive Placenta (EW-AIP) and are as follows¹¹:

With 2D Grey-scale:

• Loss of the “clear zone” (Slide 2)
• Abnormal placenta lacunae (Slide 3)
• Bladder wall interruption (Slide 4)
• Myometrial thinning (Slide 5)
• Placental budge (Slide 6)
• Focal exophytic mass (Slide 7)

With Colour Doppler imaging:

• Uterovesical hypervascularity (Slide 8)
• Subplacental hypervascularity (Slide 9)
• Bridging vessels (Slide 10)
• Placental lacunae feeder vessels (Slide 11)
• 3D intraplacental hypervascularity 
   

Sonographic screening for placenta accreta spectrum disorders focuses on the of association with low-lying or previa placenta and previous cesarean delivery. At the mid-trimester examination for fetal anomalies, all women should be asked if they have had a previous cesarean delivery. If they have, this should prompt careful assessment of the placental implantation site and the ultrasound scan should be performed with a full bladder. If the placenta is found to be low lying or previa the women should be referred to a centre specialising in prenatal diagnosis of placenta accreta spectrum disorders. 

Magnetic Resonance Imaging is widely used to assist with the prenatal diagnosis of PAS. The MRI signs include abnormal uterine bulging (Slide 12), dark intra-placental bands on T2-weighted imaging (Slide 13), heterogeneous signal intensity within the placenta, disorganised placental vasculature and disruption of the uteroplacental zone. Using these signs, the sensitivity and specificity of MRI varies between 75 and 100% and 65 and 100% respectively.⁷ Recent studies suggest that the diagnostic value of ultrasound imaging and MRI in detecting placenta accreta is comparable.^{12, 13}. However, MRI is especially useful where evaluation of uteroplacental interface is difficult, for example when there is a posterior placenta. 

Management depends both on the type of PAS and the standard practices of the specialist centre. Delivery is generally by planned caesarean section at around 34-36 weeks’ gestation, in order to avoid emergency delivery. Vaginal delivery is also performed in some specialist centres where appropriate.
When the diagnosis of abnormally invasive placenta is confirmed at the time of delivery, extirpation is not recommended as major hemorrhage may occur. The two most common management strategies are caesarean-hysterectomy and leaving the placenta in situ (conservative management). When conservative management is undertaken there is a risk of bleeding and infection during the post-operative period. However, future fertility is potentially preserved. Another strategies which is increasingly being used is partial uteroplacental resection (uterine preserving surgery) but this should only be attempted if there is no invasion into the cervix or parametrium.
 

 1. Jauniaux E, Collins SL, Jurkovic D, Burton GJ. Accreta placentation: a systematic review of prenatal ultrasound imaging and grading of villous invasiveness. Am J Obstet Gynecol. déc 2016;215(6):712‑21. 
2. Jauniaux E, Collins S, Burton GJ. Placenta accreta spectrum: pathophysiology and evidence-based anatomy for prenatal ultrasound imaging. Am J Obstet Gynecol. 2018;218(1):75‑87. 
3. Silver RM. Abnormal Placentation: Placenta Previa, Vasa Previa, and Placenta Accreta. Obstet Gynecol. sept 2015;126(3):654‑68. 
4. Fitzpatrick KE, Sellers S, Spark P, Kurinczuk JJ, Brocklehurst P, Knight M. Incidence and risk factors for placenta accreta/increta/percreta in the UK: a national case-control study. PloS One. 2012;7(12):e52893. 
5. Jauniaux E, Bhide A. Prenatal ultrasound diagnosis and outcome of placenta previa accreta after cesarean delivery: a systematic review and meta-analysis. Am J Obstet Gynecol. 2017;217(1):27‑36. 
6. Jauniaux E, Burton GJ. Placenta accreta spectrum: a need for more research on its aetiopathogenesis. BJOG Int J Obstet Gynaecol. 27 févr 2018; 
7. Jauniaux E, Chantraine F, Silver RM, Langhoff-Roos J, FIGO Placenta Accreta Diagnosis and Management Expert Consensus Panel. FIGO consensus guidelines on placenta accreta spectrum disorders: Epidemiology. Int J Gynaecol Obstet Off Organ Int Fed Gynaecol Obstet. mars 2018;140(3):265‑73. 
8. Al-Khan A, Gupta V, Illsley NP, Mannion C, Koenig C, Bogomol A, et al. Maternal and fetal outcomes in placenta accreta after institution of team-managed care. Reprod Sci Thousand Oaks Calif. juin 2014;21(6):761‑71. 
9. Eller AG, Bennett MA, Sharshiner M, Masheter C, Soisson AP, Dodson M, et al. Maternal morbidity in cases of placenta accreta managed by a multidisciplinary care team compared with standard obstetric care. Obstet Gynecol. févr 2011;117(2 Pt 1):331‑7. 
10. Shamshirsaz AA, Fox KA, Salmanian B, Diaz-Arrastia CR, Lee W, Baker BW, et al. Maternal morbidity in patients with morbidly adherent placenta treated with and without a standardized multidisciplinary approach. Am J Obstet Gynecol. févr 2015;212(2):218.e1‑9. 
11. Collins SL, Ashcroft A, Braun T, Calda P, Langhoff-Roos J, Morel O, et al. Proposal for standardized ultrasound descriptors of abnormally invasive placenta (AIP). Ultrasound Obstet Gynecol Off J Int Soc Ultrasound Obstet Gynecol. mars 2016;47(3):271‑5. 
12. D’Antonio F, Iacovella C, Palacios-Jaraquemada J, Bruno CH, Manzoli L, Bhide A. Prenatal identification of invasive placentation using magnetic resonance imaging: systematic review and meta-analysis. Ultrasound Obstet Gynecol Off J Int Soc Ultrasound Obstet Gynecol. juill 2014;44(1):8‑16. 
13. Meng X, Xie L, Song W. Comparing the diagnostic value of ultrasound and magnetic resonance imaging for placenta accreta: a systematic review and meta-analysis. Ultrasound Med Biol. nov 2013;39(11):1958‑65. 

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