A bilobed placenta, where the placenta develops as two roughly equal lobes instead of one disc, is generally not dangerous on its own. It occurs in roughly 2 to 4 percent of pregnancies, and most of those pregnancies end with healthy deliveries. The real concern is not the shape itself but a handful of complications that become more likely when the placenta splits into two lobes, particularly a condition called vasa previa. Understanding these risks and how they are monitored makes the difference between a worrying ultrasound finding and a well-managed pregnancy.
How Common Is a Bilobed Placenta
Estimates of how often bilobed placentas occur vary depending on how and when they are detected. A study of more than 6,400 pregnancies found a prenatal diagnosis rate of about 2 percent using ultrasound imaging.1PubMed. Prenatal diagnosis of bilobate placenta: incidence, risk factors and impact on pregnancy outcomes An older pathological study that examined placentas after delivery reported a rate closer to 4.2 percent, or roughly 1 in 357 deliveries.2SAGE Journals (Journal of Diagnostic Medical Sonography). Perinatal Evaluation of the Bilobed or Bipartite Placenta The gap likely reflects that many bilobed placentas go unnoticed on routine scans and are only identified once the placenta is physically inspected after birth. Either way, it is uncommon but far from rare, and most obstetricians encounter it regularly.
Why the Shape Matters for Blood Vessel Placement
The placenta’s job is to connect the fetus to the mother’s blood supply through the umbilical cord. In a normal single-disc placenta, the cord usually inserts somewhere near the center of the disc, and its blood vessels fan out across the placental tissue where they are protected. When the placenta has two lobes, the cord has to reach both of them. That means blood vessels sometimes run across the membranes between the lobes, unprotected by any placental tissue. If those unprotected vessels happen to cross over or near the cervix, the condition is called vasa previa, and it is the most clinically significant risk tied to a bilobed placenta.
Vasa previa is dangerous because when the membranes rupture during labor, those exposed vessels can tear. The blood lost is the baby’s, not the mother’s, so even a small amount of bleeding can be life-threatening to the fetus very quickly. The good news is that when vasa previa is identified before labor begins, outcomes are dramatically better because delivery can be planned by cesarean section before the membranes have a chance to rupture.
How Strong Is the Link to Vasa Previa
A systematic review and meta-analysis pooling data across multiple studies found that about a third of vasa previa cases involved a bilobed or succenturiate (accessory-lobed) placenta.3PubMed. Vasa previa and associated risk factors: a systematic review and meta-analysis That does not mean a third of bilobed placentas develop vasa previa. It means that when vasa previa does occur, the placenta frequently has an irregular shape. The absolute risk to any individual pregnancy with a bilobed placenta is much lower, because vasa previa itself is quite rare overall. Still, the association is strong enough that clinical guidelines treat a bilobed placenta as a reason to specifically look for vasa previa on ultrasound.
Other recognized risk factors for vasa previa include low-lying placentas or placenta previa, velamentous cord insertion (where the cord attaches to the membranes rather than the placental tissue), IVF pregnancies, and twin or higher-order pregnancies.4PubMed Central. Vasa Previa A bilobed placenta can overlap with several of these factors in the same pregnancy. For instance, an IVF pregnancy with a bilobed placenta and a velamentous cord insertion has multiple risk factors stacking together, and the clinical team will monitor that pregnancy more closely as a result.
Fetal Growth Concerns
Beyond vasa previa, there is evidence linking irregular placental shapes to slower fetal growth. Research on the pathophysiology of growth restriction has found that abnormal placental shapes, particularly those with irregular outlines like bilobed and extrachorial placentas, are associated with poorer obstetric outcomes and restricted fetal growth.5American Journal of Obstetrics and Gynecology. Pathophysiology of placental-derived fetal growth restriction The thinking is that a placenta split into two lobes may not transfer nutrients quite as efficiently as one with a single, well-organized disc. The blood supply has to be distributed across two separate masses connected by vessels, and that arrangement may introduce inefficiencies.
That said, the association between bilobed placentas and growth restriction is statistical, not inevitable. Many babies with bilobed placentas grow perfectly normally. Providers typically track fetal growth through serial ultrasound measurements in the second and third trimesters. If growth slows or falls below expected curves, the clinical team can intervene with closer monitoring or earlier delivery planning.
Other Pregnancy Complications Linked to the Bilobed Shape
Several other complications show up more often in pregnancies with bilobed placentas, though none of them is common enough to cause alarm on its own:
- First-trimester bleeding: The bilobed shape may be associated with early bleeding episodes, possibly because the implantation process is less straightforward when the placenta develops in two masses.
- Placental abruption: A partial or complete separation of the placenta from the uterine wall before delivery can occur, and bilobed placentas may be somewhat more prone to this because the connection between the two lobes introduces structural vulnerability.
- Retained placenta: After delivery, one or both lobes may not detach cleanly. If a lobe or fragment stays behind, it can cause heavy bleeding or infection and may require manual removal or a procedure to clear the uterus.
These complications have been described in clinical literature on bilobed placentas, with retained placenta being a particularly well-known concern.2SAGE Journals (Journal of Diagnostic Medical Sonography). Perinatal Evaluation of the Bilobed or Bipartite Placenta The retained placenta risk is worth knowing about because it affects what happens after birth, not just before. The delivery team will typically inspect the placenta carefully to confirm that both lobes and any connecting tissue have been fully delivered. When a bilobed placenta is known about in advance, this inspection is routine and straightforward.
The IVF Connection
If you conceived through in vitro fertilization or another form of assisted reproductive technology, a bilobed placenta is more likely. Research has found that women who conceived via ART had roughly three-fold higher odds of developing a bilobed placenta compared with spontaneous conceptions.6PubMed Central. The Association of Assisted Reproductive Technology with Placental and Umbilical Abnormalities IVF pregnancies also show higher rates of low-lying placentas, placenta previa, and velamentous cord insertion.7PubMed. Ultrasound diagnosis of placental and umbilical cord anomalies in singleton pregnancies resulting from in-vitro fertilization
The reasons are not completely understood, but the leading theory involves how and where the embryo implants. In natural conception, the embryo floats through the uterine cavity and implants where conditions are optimal. In IVF, the embryo is transferred directly into the uterus, and it may implant in a slightly different location or orientation. It is also possible that the laboratory culture conditions or the embryo transfer process itself influence how the placenta develops in those first critical days. Whatever the cause, IVF pregnancies are often monitored more closely for placental irregularities as a matter of course, so a bilobed placenta in an IVF pregnancy is likely to be caught early.
How a Bilobed Placenta Is Detected and Managed
Most bilobed placentas are identified on a routine second-trimester anatomy scan, the detailed ultrasound performed around 18 to 22 weeks. The sonographer may notice two distinct masses of placental tissue instead of one. Sometimes the finding is subtle enough that it requires a follow-up scan to confirm. In other cases, it is not detected until delivery, when the placenta is physically inspected.
Once a bilobed placenta is confirmed on imaging, clinical guidelines recommend a targeted follow-up approach. This includes checking where the lobes sit relative to the cervix, evaluating where the umbilical cord inserts, and looking specifically for any vessels crossing the lower uterine segment that might indicate vasa previa.8Italian Journal of Gynæcology & Obstetrics. Bilobed placenta and management choices: literature review and two cases studies Color Doppler ultrasound, which visualizes blood flow, is particularly useful for mapping where vessels run between the two lobes. In some circumstances, MRI imaging provides additional detail, and case reports describe its use in confirming vasa previa when ultrasound findings were ambiguous.9PubMed. Clinical significances of magnetic resonance imaging in prenatal diagnosis of vasa previa in a woman with bilobed placentas
Management recommendations emphasize that a dedicated team experienced with placental abnormalities should oversee the pregnancy when a bilobed placenta is found, particularly when additional risk factors are present.8Italian Journal of Gynæcology & Obstetrics. Bilobed placenta and management choices: literature review and two cases studies In practice, this usually means the pregnancy is managed at a hospital with maternal-fetal medicine specialists and neonatal care capabilities, rather than at a smaller community facility. If no vasa previa or other complicating factor is identified, vaginal delivery is typically considered safe. If vasa previa is found, a planned cesarean section before the onset of labor is the standard recommendation.
What Happens After Delivery
The postpartum period carries its own set of considerations when the placenta has two lobes. The main concern is making sure the entire placenta has been delivered. With a single-disc placenta, incomplete delivery is usually obvious because the disc looks torn or incomplete. With a bilobed placenta, one lobe might separate and deliver while the other remains attached to the uterine wall. A retained lobe can cause serious bleeding (postpartum hemorrhage) or lead to infection if it is not identified and removed promptly.
Examining the delivered placenta carefully is standard obstetric practice, but it takes on added importance with bilobed placentas. The clinical team looks for the vascular connection between the two lobes and confirms that both masses are present and intact.10PubMed Central. Succenturiate Placental Lobe Abruption If there is any concern that placental tissue may have been left behind, an ultrasound of the uterus can be performed immediately after delivery, and manual removal or a uterine procedure can be done if needed. When the bilobed shape was known about before delivery, the team is already watching for this, which substantially reduces the risk of a missed retained lobe.
Bilobed Versus Succenturiate Placentas
You may come across the term “succenturiate lobe” when reading about placental variations, and it is worth knowing the difference. A bilobed placenta has two lobes that are roughly equal in size, both contributing meaningfully to the placenta’s function. A succenturiate placenta has one main mass and a much smaller accessory lobe, connected by vessels running through the membranes. Both conditions share several of the same risks, including vasa previa, velamentous cord insertion, and retained placental tissue. In clinical practice, the management approach for both is similar: check where the vessels run, rule out vasa previa, monitor fetal growth, and inspect the placenta thoroughly after delivery.
The two conditions are sometimes grouped together in research studies, which can make it hard to tease out whether one carries a distinctly different risk profile than the other. Most clinicians treat them as part of the same family of placental shape variations that require the same kind of vigilant follow-up.
The Emotional Weight of a Placental Diagnosis
Getting told that something is “abnormal” about your placenta during a pregnancy ultrasound can be genuinely frightening, even when the clinical reality is manageable. Research on placenta previa, a different but related placental complication, found that roughly half of diagnosed patients met screening criteria for depression, and a similar proportion showed elevated anxiety and post-traumatic stress symptoms compared with those who had uncomplicated pregnancies.11PubMed Central. Placenta Previa is Associated with Maternal Psychological Distress: A Mediation Analysis of Depression, Anxiety, and Post-Traumatic Stress Symptoms There is no equivalent study specifically on the psychological impact of a bilobed placenta diagnosis, but it stands to reason that any finding labeled “abnormal” during pregnancy can trigger significant worry.
The reality is that most bilobed placentas are an incidental finding that leads to some extra monitoring and not much else. If your provider frames it as something to keep an eye on rather than something to panic about, that framing is probably accurate. The complications described in this article are possibilities, not certainties, and most of them are highly manageable when detected. Asking your provider to explain specifically what they are watching for, and what would trigger a change in your care plan, can help turn a vague sense of worry into something more concrete and controllable.
When a Bilobed Placenta Actually Changes Your Care Plan
For many pregnancies with a bilobed placenta and no other complicating factors, the practical changes to prenatal care are modest: an extra scan or two to check cord insertion and vessel location, a closer look at fetal growth in the third trimester, and a heads-up to the delivery team about the placental shape. You may not need to change hospitals, see a specialist, or plan a cesarean section.
The situations where a bilobed placenta genuinely changes the game are when it is accompanied by something else. Vasa previa is the most important, because it shifts delivery planning entirely toward a scheduled cesarean. A low-lying lobe that partially covers the cervix raises similar concerns. Velamentous cord insertion, where the cord meets the membranes rather than the placental tissue directly, adds another layer of monitoring. And when fetal growth starts to slow, the clinical team may recommend earlier delivery. Each of these scenarios is individually uncommon, and most pregnancies with a bilobed placenta will not encounter any of them. But the combination of a bilobed placenta with one or more of these factors is exactly why clinical teams take the diagnosis seriously enough to look carefully and plan ahead.