A thin decidua can also be a contributing factor to such trophoblastic invasion. Some studies suggest that the rate of incidence is higher when the fetus is female. Other risk factors include low-lying placenta, anterior placenta, congenital or acquired uterine defects (such as uterine septa), leiomyoma, ectopic implantation of placenta (including cornual pregnancy).Pregnant women above 35 years of age who have had a caesarian section and now have a placenta previa overlying the uterine scar have a 40% chance of placenta accreta. Pathogenesis
The placenta forms an abnormally firm and deep attachment to the uterine wall. There is absence of the decidua basalis and incomplete development of the Nitabuchs layer. There are three forms of placenta accreta, distinguishable by the depth of penetration. Diagnosis
When the antepartum diagnosis of placenta accreta is made, it is usually based on ultrasound findings in the second or third trimester. Sonographic findings that may be suggestive of placenta accreta include:

Loss of normal hypoechoic retroplacental zone
Multiple vascular lacunae (irregular vascular spaces) within placenta, giving "Swiss cheese" appearance
Blood vessels or placental tissue bridging uterine-placental margin, myometrial-bladder interface, or crossing the uterine serosa
Retroplacental myometrial thickness of <1 mm
Numerous coherent vessels visualized with 3-dimensional power Doppler in basal viewUnfortunately, the diagnosis is not easy and is affected by a significant interobserver variability. In doubtful cases it is possible to perform a nuclear magnetic resonance (MRI) of the pelvis, which has a very good sensitivity and specificity for this disorder.
Placenta accreta occurs when all or part of the placenta attaches abnormally to the myometrium (the muscular layer of the uterine wall). Three grades of abnormal placental attachment are defined according to the depth of attachment and invasion into the muscular layers of the uterus:

Accreta – chorionic villi attached to the myometrium, rather than being restricted within the decidua basalis. Increta – chorionic villi invaded into the myometrium. Percreta – chorionic villi invaded through the perimetrium (uterine serosa).Because of abnormal attachment to the myometrium, placenta accreta is associated with an increased risk of heavy bleeding at the time of attempted vaginal delivery. The need for transfusion of blood products is frequent, and surgical removal of the uterus (hysterectomy) is sometimes required to control life-threatening bleeding.Rates of placenta accreta are increasing. As of 2016, placenta accreta affects an estimated 1 in 272 pregnancies. Risk factors
An important risk factor for placenta accreta is placenta previa in the presence of a uterine scar. Placenta previa is an independent risk factor for placenta accreta. Additional reported risk factors for placenta accreta include maternal age and multiparity, other prior uterine surgery, prior uterine curettage, uterine irradiation, endometrial ablation, Asherman syndrome, uterine leiomyomata, uterine anomalies, and smoking. The condition is increased in incidence by the presence of scar tissue i.e. Ashermans syndrome usually from past uterine surgery, especially from a past dilation and curettage, (which is used for many indications including miscarriage, termination, and postpartum hemorrhage), myomectomy, or caesarean section.