When an ultrasound reveals what looks like “two placentas” while carrying only one baby, the finding can be confusing. This visual appearance does not mean you are having twins, nor does it typically indicate a serious problem. What is observed is an anatomical variation where the placenta, the temporary organ responsible for nutrient and gas exchange, has developed into two distinct sections. These variations are relatively uncommon (2% to 8% of pregnancies), and with appropriate monitoring, most result in healthy outcomes. This unique shape requires specific attention from your care team.
Understanding Bilobed and Succenturiate Placentas
The term “two placentas” in a singleton pregnancy usually refers to one of two morphological variations: a bilobed or a succenturiate placenta. A bilobed, or bipartite, placenta is a single organ that has developed into two lobes of nearly equal size. These two sections are separated by a membrane but remain connected by a bridge of chorionic tissue containing blood vessels.
In contrast, a succenturiate lobe placenta involves a main placental body and one or more smaller, accessory lobes. The smaller lobe is physically separate from the main disc, but critical fetal blood vessels run through the membranes to connect it. The difference in size is the primary distinction between a bilobed and a succenturiate placenta. In both cases, the entire structure is a single functioning unit supplying one fetus.
Factors Contributing to Placental Variations
The exact mechanism causing these variations is not entirely understood, but they are thought to arise early in pregnancy based on how the chorionic villi spread. One leading hypothesis, called the dynamic placentation theory, suggests that localized atrophy, or poor development, occurs in part of the uterine wall. This poor vascularization or decidualization causes the placenta to spread out and develop in two separate areas where the uterine lining is more receptive.
Certain maternal factors are associated with an increased likelihood of these placental shapes. Advanced maternal age and the use of assisted reproductive technology (ART), such as in vitro fertilization, are known risk factors. Additionally, the site of implantation plays a role; if the embryo implants over a pre-existing condition, such as a uterine septum or a fibroid, the placenta may be forced to grow in a divided manner.
Associated Risks for Mother and Baby
While many pregnancies with these placental variations proceed without complication, the altered structure introduces specific risks that require closer observation. One primary concern for the baby is the risk of vasa previa. This occurs when the connecting blood vessels running between the two lobes pass across the internal cervical opening. If the membranes rupture during labor, these exposed fetal vessels may tear, leading to rapid fetal blood loss.
Another related risk is an abnormal umbilical cord insertion, such as a velamentous cord insertion, which is more common with bilobed placentas. In this condition, the umbilical cord vessels travel unprotected within the fetal membranes before reaching the placental tissue. For the birthing parent, the primary concern occurs during the third stage of labor, after delivery of the baby. The separate lobe, especially the smaller succenturiate one, may fail to detach from the uterine wall and be retained. Retained placental tissue prevents the uterus from contracting properly, which is a common cause of postpartum hemorrhage (PPH).
Monitoring and Management Strategies
The diagnosis of a bilobed or succenturiate placenta, usually made via prenatal ultrasound, allows medical teams to implement proactive management strategies. The most immediate step is increased surveillance using ultrasound and color Doppler imaging. This focused scanning is used specifically to visualize the connecting vessels and confirm whether they cross the cervix, which diagnoses vasa previa.
If vasa previa is confirmed, a planned Cesarean delivery is typically scheduled before labor begins to prevent the rupture of the exposed vessels. For those without vasa previa, the delivery plan may remain vaginal, but the care team prepares for the possibility of retained tissue. After the baby is born, the provider performs a careful manual inspection of the delivered placenta to confirm that all lobes have been expelled from the uterus. With careful monitoring and management, the potential risks associated with these placental variations can be minimized.