The placenta is a complex, temporary organ that provides the necessary interface for gas, nutrient, and waste exchange between a pregnant person and the developing fetus. While this organ typically forms as a single, disc-shaped mass attached to the uterine wall, structural variations are relatively common. An extra lobe, often called an accessory lobe, is a significant structural anomaly. These variations arise from deviations in how the placenta establishes itself early in pregnancy. Understanding these structures is important for anticipating potential complications during the final stages of pregnancy and delivery.
Defining Accessory Placental Structures
The term “accessory placental structures” refers to instances where the placental tissue is divided into two or more distinct masses, rather than a single, unified disc. The two most frequent forms are the bilobate placenta and the succenturiate lobe. A bilobate, or bipartite, placenta, is characterized by two nearly equal-sized lobes, often bridged by thin, vascularized tissue where the umbilical cord inserts.
The succenturiate lobe is anatomically distinct, featuring one or more smaller accessory cotyledons separate from the main placental body. These smaller lobes are connected to the main mass solely by blood vessels traversing the intervening fetal membranes. This vascular connection differs from the tissue bridge found in a bilobate placenta. If an accessory lobe develops without any connecting fetal blood vessels, it is termed a placenta spuria.
Developmental Origin of Extra Lobes
The formation of an extra placental lobe is directly linked to an aberration in the early development of the chorion, the outermost fetal membrane. In a typical pregnancy, the chorion differentiates into two distinct regions. The chorion frondosum is the portion where villi persist and proliferate, establishing the main body of the placenta.
The remaining portion, the chorion laeve, is where the villi normally atrophy and regress, resulting in a smooth, non-functional membrane. An accessory lobe develops when villi in a localized area of the chorion laeve fail to regress and instead persist, remaining metabolically active. This persistence allows a separate, functional cotyledon to form distant from the primary implantation site. The resulting size discrepancy determines if the structure is classified as a bilobate placenta or a smaller succenturiate lobe.
Factors Increasing Development Risk
While the precise cause of the developmental error is not fully known, several maternal and pregnancy-related conditions are associated with an increased risk of accessory lobe formation. Advanced maternal age is a factor, suggesting that changes in the uterine environment influence placental development. Pregnancies conceived using assisted reproductive technologies, such as in vitro fertilization (IVF), also show a higher incidence of these structural anomalies.
Conditions that alter the surface of the uterine lining, or endometrium, contribute to this risk. This includes cases where the placenta implants over existing structural abnormalities, such as uterine leiomyomas (fibroids). A history of previous uterine surgery, including prior cesarean deliveries, may also predispose the tissue to form an accessory lobe. These factors create an environment where developing chorionic villi persist in areas they normally would not, leading to the split morphology.
Implications During Pregnancy and Delivery
The presence of an accessory placental lobe is typically identified during a routine second-trimester ultrasound, allowing clinicians to plan for potential complications. A primary concern during pregnancy is vasa previa, a condition where the connecting blood vessels between the lobes cross directly over the cervical opening. If these unsupported vessels rupture during labor, it can lead to rapid and severe fetal blood loss.
Other antenatal risks include a higher likelihood of placental abruption, where the placenta prematurely separates from the uterine wall. The most frequent complication occurs after delivery: if the accessory lobe fails to detach and is retained in the uterus after the main placenta is expelled, it can cause immediate or delayed postpartum hemorrhage.
The retained lobe prevents the uterus from fully contracting, which is the body’s natural mechanism for stopping blood flow at the placental site. When an accessory lobe is diagnosed antenatally, the delivered placenta must be inspected to ensure all parts have been expelled. If any portion is missing, immediate intervention, such as manual removal or surgery, is necessary to prevent excessive blood loss and maternal infection.