The placenta is a temporary, discoid-shaped organ that develops within the uterus during pregnancy, serving as the interface between the mother and the fetus. It facilitates the exchange of oxygen, nutrients, and waste products through the maternal circulation, performing the functions of respiration, nutrition, and excretion for the developing baby. The organ also produces hormones, such as human chorionic gonadotropin, necessary for maintaining the pregnancy. Placental abnormalities are conditions where the organ develops, implants, or functions abnormally, potentially leading to complications for both the mother and the baby.
Categorizing Placental Abnormalities
Placental problems are often grouped into three main categories based on the nature of the deviation from normal development. The first involves the location where the placenta implants itself within the uterus, particularly in relation to the cervix. The second focuses on the depth of the attachment, specifically how firmly the placenta adheres to the uterine wall. Finally, the third category encompasses issues related to the overall morphology, or physical structure, of the placenta and the insertion point of the umbilical cord.
Abnormalities of Deep Attachment (The Accreta Spectrum)
The most significant group of placental abnormalities involves the depth of attachment, referred to as the Placenta Accreta Spectrum (PAS). This condition occurs when the placental tissue attaches too deeply to the myometrium (the muscular layer of the uterine wall) due to a defect in the normal barrier layer. The incidence has increased significantly, now estimated to affect approximately 1 in 272 births, a rise linked closely to the increased rate of cesarean deliveries.
The spectrum is divided into three levels of increasing invasion into the uterus. The least invasive form is Placenta Accreta, where the placental villi attach directly to the myometrium without penetrating it. Placenta Increta is the next level, where the villi invade into the uterine muscle. The most severe form, Placenta Percreta, occurs when the villi penetrate through the entire thickness of the uterine wall, sometimes invading nearby organs like the bladder or rectum.
The danger of the Accreta Spectrum lies in the failure of the placenta to detach spontaneously after birth. Attempting to remove an abnormally adhered placenta can cause massive maternal hemorrhage. The average blood loss in these cases is approximately three liters, frequently requiring extensive blood transfusions. To prevent this bleeding, the primary management often involves a planned cesarean delivery followed immediately by a hysterectomy (surgical removal of the uterus).
Abnormalities of Position and Structure
The position of the placenta relative to the cervix is a common abnormality, with Placenta Previa being the most recognized condition. This occurs when the placenta implants low in the uterus, partially or completely covering the internal cervical opening. Previa is a major cause of third-trimester bleeding and typically necessitates a cesarean delivery to avoid severe maternal hemorrhage when the cervix dilates.
Structural abnormalities of the placenta or umbilical cord connection can directly impact fetal circulation. Velamentous Cord Insertion is a structural abnormality where the umbilical cord attaches to the fetal membranes instead of directly into the center or edge of the placenta. The blood vessels must then travel unprotected by Wharton’s jelly, the connective tissue that normally cushions the cord, before reaching the placental disc. This lack of protection makes the vessels vulnerable to compression or tearing, which can lead to rapid fetal blood loss.
A condition known as Vasa Previa is a particularly hazardous consequence of a velamentous insertion or a succenturiate lobe. Vasa Previa means that these unprotected fetal blood vessels cross over or lie near the opening of the cervix. If the membranes rupture, or if the cervix dilates during labor, these exposed vessels can tear, resulting in acute and potentially fatal fetal hemorrhage.
A Succenturiate Lobe is a morphological variation where one or more smaller, accessory placental lobes develop in the membranes, separate from the main placental body. These smaller lobes are connected to the main placenta only by blood vessels that traverse the membranes. Risks include the connecting vessels lying over the cervix (causing Vasa Previa) or the accessory lobe being retained inside the uterus after delivery, increasing the risk of postpartum hemorrhage.
Identifying Risk Factors and Management
Several factors increase the likelihood of developing placental abnormalities. The most important risk factor for the Accreta Spectrum is a history of a previous cesarean delivery combined with a current diagnosis of placenta previa. The risk of Accreta increases progressively with each subsequent cesarean section.
Other common risk factors include:
- Advanced maternal age (typically over 35 years).
- Multiple previous pregnancies (multiparity).
- A history of any uterine surgery, such as myomectomy or curettage.
- Smoking.
- Pregnancies achieved through assisted reproductive technology.
These conditions are typically identified through routine prenatal care, most commonly during the detailed second-trimester ultrasound scan. Ultrasound evaluation, often supplemented with Doppler flow studies, allows providers to visualize the placental location and structure in relation to the cervix and uterine wall. When an abnormality is suspected, management focuses on minimizing risk for both the mother and the baby. This often involves careful monitoring, planned early delivery via cesarean section, and ensuring delivery takes place at a facility equipped with specialized surgical teams to manage complications like massive hemorrhage.