Placenta accreta spectrum (PAS), also called morbidly adherent placenta is a medical condition that occurs when all or part of the placenta attaches abnormally to the myometrium (the muscular layer of the uterine wall) during pregnancy. This condition was first documented in medical literature in 1927. Three grades are defined by the International Federation of Gynecology and Obstetrics (FIGO), which were initially named accreta, increta, and percreta in order of severity. Because of abnormal attachment to the myometrium, PAS is associated with an increased risk of massive hemorrhaging, heavy bleeding, at the time of attempted vaginal delivery. This leads many to deliver through a caesarean section. The need for transfusion of blood products is frequent, and a surgical removal of the uterus (hysterectomy) is sometimes required to control life-threatening bleeding. Rates of placenta accreta are increasing, and are even higher in developing countries. A 2016 study found that placenta accreta affected 1 in 272 women with a birth-related hospital discharge diagnosis in the US. Furthermore, the increase in PAS prevalence over decades has been a major cause of morbidity and mortality among pregnant women, and has been a main factor in the increase of caesarean deliveries.
Pathogenesis The pathogenesis of PAS includes the formation of an abnormally firm and deep attachment to the uterine wall by the placenta. In addition, there may be an absence of the decidua basalis and incomplete development of the Nitabuch's layer. Below are the three forms of placenta accreta spectrum, distinguishable by the depth of penetration into the uterine wall.
Women experience higher morbidity with placenta percreta compared to placenta accreta and increta. In cases of placenta percreta, where the uterus is deeply penetrated into and through the myometrium to the bladder or rectum, it is highly advised to avoid any attempts of removing the placenta. Leaving the placenta in situ, not removing it after childbirth, has been part of the conservative management of PAS discussed later.
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 hypo-echoic 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 view The diagnosis is not easy and is affected by a significant interobserver variability; failure to diagnose and misdiagnosis are common. 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. MRI findings associated with placenta accreta include dark T2 bands, bulging of the uterus, and loss of the dark T2 interface. Although there are isolated case reports of placenta accreta being diagnosed in the first trimester or at the time of abortion < 20 weeks' gestational age, the predictive value of first-trimester ultrasound for this diagnosis remains unknown. Women with a placenta previa or "low-lying placenta" overlying a uterine scar early in pregnancy should undergo follow-up imaging in the third trimester with attention to the potential presence of placenta accreta. Despite the difficulty in diagnosing PAS, there exists many risk factors that can aid in the diagnosis.
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. Any anomaly in the uterine wall, whether superficial or deep, can lead to PAS, as that anomaly assists the blastocyst to implant onto the uterine wall at that location. Incidence of the condition is increased by the presence of scar tissue such as Asherman's syndrome from past uterine surgery, especially from a past dilation and curettage, (which is used for many indications including miscarriage, induced abortion, and postpartum hemorrhage), myomectomy, or caesarean section. Caesarean deliveries have been deemed the main factor for women to develop PAS and placenta previa due to the tissue scarring from the delivery. Almost a third of all births in the United States are by caesarean section. A systemic review found that caesarean deliveries increased the incidence of placenta previa. Furthermore, women who had placenta previa and at least 3 caesarean deliveries were statistically more likely to develop placenta accreta than those with previa and no history of caesarean deliveries. 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, which comes with any complications.
… excerpt ends here. Continue reading the full article.
