What Is the Banana Sign on Fetal Ultrasound?

The banana sign is an ultrasound finding seen during prenatal imaging in which the fetal cerebellum appears curved into a crescent shape, resembling a banana. It is one of the most reliable indirect markers of open spina bifida, a condition where part of the spinal cord and its surrounding structures do not close properly during early development. First described in the mid-1980s, the sign remains a cornerstone of second-trimester screening because it shifts the sonographer’s attention from the spine, where small defects can be hard to spot, to the brain, where the abnormality is often easier to see.

What the Sign Actually Looks Like on Ultrasound

During a routine anatomy scan, the sonographer obtains a cross-sectional view of the fetal skull at a level that shows the cerebellum, the structure at the base of the brain responsible for coordination. In a healthy fetus, the two cerebellar hemispheres appear as a rounded, dumbbell-shaped structure sitting in the posterior fossa, with a fluid-filled space called the cisterna magna visible behind them. When the banana sign is present, those hemispheres are compressed and pushed forward, curving into a concave arc. At the same time, the cisterna magna is obliterated, meaning the normal pocket of fluid behind the cerebellum essentially disappears. The resulting image on the ultrasound screen looks strikingly like a banana in cross-section. In the original description of the sign, twelve of twenty-one fetuses with spina bifida showed this curved cerebellar appearance with simultaneous loss of the cisterna magna.¹

Why the Cerebellum Changes Shape

The banana sign is not a random deformity. It is a direct consequence of a chain of events triggered by the open spinal defect. When the neural tube fails to close, cerebrospinal fluid leaks out through the gap in the spine. This leakage reduces the normal fluid pressure inside the developing skull, and the posterior fossa, the compartment at the base of the skull where the cerebellum sits, does not expand the way it should. The result is a condition called Chiari II malformation, in which the cerebellum and parts of the brainstem are pushed downward through the opening at the base of the skull. That downward herniation compresses the cerebellar hemispheres against each other and against the skull, producing the curved banana shape visible on ultrasound.

Research on fetal brain development in Chiari II has shown that the structural changes go beyond the cerebellum itself. Affected fetuses have significantly smaller volumes of the diencephalon, a central brain region involved in relaying sensory information, along with substantially larger lateral ventricles and expanded proliferative zones compared to unaffected fetuses.² In other words, the banana sign visible on ultrasound is the surface expression of a broader set of brain changes driven by altered fluid dynamics in the skull.

How Reliable Is It as a Marker for Spina Bifida

The banana sign has consistently performed well as an indirect indicator of open neural tube defects. In an early validation study, twenty-two of twenty-three fetuses with confirmed neural tube defects displayed the sign.³ A large German multicentre study examining cases between eighteen and twenty-two weeks of gestation found the banana sign in about 97% of fetuses with spina bifida.⁴ The false-positive rate has historically been reported as essentially zero, meaning that when the sign appears, it almost always indicates a real problem.⁵

That near-zero false-positive rate is part of what makes the sign so useful in practice. A sonographer who sees a banana-shaped cerebellum knows to examine the fetal spine with extra care, even if the spinal defect itself is small or in a location that is difficult to image directly. The sign effectively functions as a red flag that redirects the clinical workup.

The Lemon Sign and How It Differs

The banana sign is almost always discussed alongside another ultrasound finding called the lemon sign. The lemon sign refers to a scalloped or inward-bowing appearance of the frontal bones of the fetal skull, making the head look somewhat like a lemon when viewed from above. Both signs are cranial markers of open spina bifida, but they behave differently as pregnancy progresses, and understanding that difference matters for screening.

A study of 130 fetuses with open spina bifida found that the lemon sign was present in 98% of cases before twenty-four weeks of gestation but dropped to just 13% after that point. Cerebellar abnormalities, by contrast, were present in about 95% of cases regardless of gestational age.⁶ The banana sign itself was the predominant cerebellar finding early in pregnancy, appearing in about 72% of cases before twenty-four weeks. After that point, the cerebellum was more often simply absent from the expected view, visible in about 81% of later cases as apparent cerebellar “absence” rather than the classic banana curve.⁷

The practical takeaway is that the lemon sign fades as the skull bones mature and become less pliable, while the banana sign evolves rather than disappears. Sonographers scanning later in pregnancy need to be alert to the possibility that the cerebellum may simply not be visible at all, which is itself a warning sign even though it does not look like the classic banana shape.

The Cisterna Magna as an Early Clue

Because the banana sign becomes most apparent during the second-trimester anatomy scan, typically performed around eighteen to twenty-two weeks, researchers have looked for signs that could flag a problem even earlier. One approach involves measuring the cisterna magna, the fluid space behind the cerebellum, during the first-trimester screening window around eleven to thirteen weeks. A study examining this approach found that all fetuses later confirmed to have open spina bifida had a cisterna magna width below the fifth percentile at that early stage.⁸ This makes sense given the mechanism: the fluid leakage through the open spine begins early, and the cisterna magna starts shrinking well before the cerebellar compression becomes dramatic enough to produce a clear banana shape.

An abnormally small cisterna magna at eleven to thirteen weeks is not as visually striking as the banana sign, and it is measured rather than simply observed. But it provides an earlier window for suspicion, potentially allowing earlier diagnostic workup and counseling for the parents.

When the Banana Sign Appears Alongside Ventriculomegaly

Enlargement of the brain’s fluid-filled ventricles, known as ventriculomegaly, is another common finding in fetuses with open spina bifida. It occurs for the same underlying reason: the Chiari II malformation disrupts the normal flow and drainage of cerebrospinal fluid, causing it to accumulate in the ventricles. When a sonographer detects ventriculomegaly during a scan, clinical guidance recommends a careful look at the spine, and the presence of the banana or lemon sign alongside enlarged ventricles strengthens the suspicion of a neural tube defect.⁹ The spinal defect itself may be subtle or positioned in a way that makes it hard to see directly, so the combination of cranial signs often guides the diagnosis more than the spine view alone.

What Happens After Fetal Surgery

One of the more remarkable aspects of the banana sign is that it can reverse. Open fetal surgery to repair a myelomeningocele, the most severe form of spina bifida, involves closing the spinal defect while the baby is still in the womb. This stops the cerebrospinal fluid leak, allows pressure to normalize in the skull, and gives the posterior fossa a chance to expand. A study following fifty fetuses after open fetal repair found that the hindbrain herniation resolved in 96% of cases before delivery, and the cerebellum resumed normal growth in all of them.¹⁰

The speed of resolution turned out to carry prognostic value. When the herniation disappeared within the first two weeks after surgery, the odds of needing a shunt to drain excess cerebrospinal fluid during the baby’s first year of life dropped by roughly 80%. Every fetus in the study whose herniation persisted up to delivery ultimately required a shunt.¹¹ This means that after fetal surgery, the disappearance of the banana sign on follow-up ultrasounds is not just cosmetically reassuring. It is a functional indicator of how well cerebrospinal fluid dynamics are recovering, and it helps clinicians predict which babies will need additional intervention after birth.

Rare False Positives and Diagnostic Pitfalls

While the false-positive rate for the banana sign has been described as essentially zero, extremely rare exceptions exist. At least one documented case involved a fetus with triploidy, a chromosomal abnormality involving an extra set of chromosomes, where the banana and lemon signs appeared on ultrasound and initially led to a suspected diagnosis of spina bifida that turned out to be incorrect.¹² Triploidy can cause brain abnormalities that mimic the posterior fossa changes of Chiari II, creating a look-alike on ultrasound.

This edge case illustrates why the banana sign alone is not treated as a final diagnosis. It is an alert that triggers a more detailed examination of the spine, often supplemented by additional imaging or genetic testing. If the spine appears intact despite a banana-sign appearance of the cerebellum, clinicians consider alternative explanations, including chromosomal abnormalities and other rare posterior fossa malformations. The sign’s value lies in its sensitivity as a screening tool, not as a standalone diagnostic.

Artificial Intelligence and Automated Detection

Given how visual and pattern-dependent the banana sign is, it is a natural candidate for automated detection using machine learning. Researchers have developed deep learning models trained on second-trimester fetal brain ultrasound images to recognize abnormal findings, including posterior fossa abnormalities. One such model achieved an accuracy above 93% with a sensitivity of about 93% and a specificity of about 94% on test images, classifying each image in roughly 50 milliseconds.¹³

These tools are not replacing sonographers, but they could serve as a safety net, particularly in settings where experienced fetal medicine specialists are not always available. A second pair of algorithmic “eyes” that flags a potentially abnormal cerebellum could catch cases that might otherwise be missed during a busy clinic day, giving the sonographer a prompt to take a closer look at the spine.

The Banana Sign Outside Obstetric Ultrasound

The term “banana sign” in prenatal imaging is by far the most common medical usage, but the banana shows up elsewhere in radiology’s naming conventions too. In musculoskeletal imaging, “banana fractures” refer to a specific type of stress fracture seen in Paget’s disease of bone. These are incomplete transverse fractures that occur along the convex surface of bowed long bones, particularly the lateral side of a bowed femur or the front of a bowed tibia.¹⁴ The name comes from the curved, crescent-like appearance of the fracture line against the bowed bone, but the clinical context is entirely different from the prenatal banana sign.

Why Radiology Uses Food Names at All

The banana sign belongs to a surprisingly large family of “food signs” in medical imaging. Radiologists have named findings after everything from lemons and bananas to ground glass and popcorn. The tradition persists because visual pattern recognition is central to the discipline, and a memorable name tied to a familiar object makes a sign easier to teach, easier to recall during a scan, and easier to communicate between colleagues.¹⁵ A term like “banana sign” immediately conveys both the shape the radiologist is looking for and the clinical context in which it matters, compressing a paragraph of anatomical description into two words.¹⁶

For parents who encounter the term during a pregnancy scan, the whimsical name can feel jarring when attached to a serious finding. But the naming convention is not meant to trivialize the condition. It exists because in a fast-paced clinical environment, a name that sticks in memory can be the difference between catching a subtle abnormality and missing it. The banana sign has likely contributed to earlier detection of spina bifida in countless pregnancies precisely because its name is vivid enough that every trainee who learns it remembers what to look for.