What causes placenta accreta

Overview

Placenta accreta is a serious pregnancy complication where the placenta implants too deeply into the uterine wall. Normally, the placenta detaches easily from the uterine wall after childbirth. In cases of placenta accreta, the placenta remains abnormally attached, leading to significant bleeding and potential complications during and after delivery. This condition is part of a spectrum known as the placenta accreta spectrum (PAS), which includes placenta increta (placenta invades the uterine muscle) and placenta percreta (placenta penetrates through the uterine wall and potentially into nearby organs).

Causes and Risk Factors

The primary cause of placenta accreta is believed to be a defect in the decidua, the specialized lining of the uterus that the placenta normally attaches to. When the decidua is compromised, the placenta may grow too deeply into the myometrium (the muscular wall of the uterus). While the exact trigger for this defective implantation is not fully understood, several factors are strongly associated with an increased risk:

Previous Uterine Surgery and Cesarean Deliveries:

This is the most significant risk factor. Scarring on the uterine wall from previous Cesarean sections (C-sections) or other uterine surgeries, such as myomectomy (removal of fibroids) or procedures to remove uterine polyps, can interfere with the normal development of the decidua. The more C-sections a woman has had, the higher her risk of developing placenta accreta. For women with one prior C-section, the risk is around 5-10%. This risk escalates considerably with subsequent C-sections, potentially reaching over 30% with four or more deliveries via C-section.

Placenta Previa:

When the placenta implants over or very close to the internal opening of the cervix, it is called placenta previa. This condition is found in approximately 50% of all cases of placenta accreta. The exact mechanism linking placenta previa and accreta is not entirely clear, but it is thought that the placenta may attempt to grow through an area of the uterus that has been previously affected by scarring or damage, often associated with prior surgical interventions.

Maternal Age:

Advanced maternal age, generally considered to be 35 years and older, is associated with an increased risk of placenta accreta. As women age, changes in the uterine lining and vascularity may play a role in the increased incidence of abnormal placental implantation.

Other Uterine Conditions:

Conditions that affect the uterine lining, such as Asherman's syndrome (intrauterine adhesions or scarring), or inflammation of the uterus (endometritis), can also increase the risk. These conditions can disrupt the normal decidual layer, making it more susceptible to abnormal placental invasion.

In Vitro Fertilization (IVF):

Some studies suggest a slightly increased risk of placenta accreta in pregnancies conceived via IVF, although this association is still being investigated and may be related to underlying fertility issues or specific protocols used.

Diagnosis

Diagnosis of placenta accreta is typically made during pregnancy through imaging techniques. Ultrasound, particularly a detailed anatomy scan, is the primary diagnostic tool. Characteristic findings include abnormal placental echogenicity, thinning or absence of the normal hypoechoic space between the uterine wall and the placenta, and the presence of vascular lacunae within the placenta that resemble "Swiss cheese." Color Doppler ultrasound can help identify abnormal blood flow patterns. Magnetic Resonance Imaging (MRI) may be used in complex cases or when ultrasound findings are inconclusive to better delineate the extent of placental invasion.

Management and Delivery

Management of placenta accreta is complex and requires a multidisciplinary team, including obstetricians, maternal-fetal medicine specialists, anesthesiologists, neonatologists, and often urologists and general surgeons. Delivery is usually recommended between 34 and 37 weeks of gestation via scheduled C-section, often with the placenta left in situ to minimize bleeding. In many cases, a hysterectomy (surgical removal of the uterus) is performed immediately after the baby is delivered to control bleeding and prevent life-threatening hemorrhage. The decision for hysterectomy is made based on the severity of the accreta and the surgeon's assessment during the procedure.