What Are Lemon & Banana Signs on Fetal Ultrasound?

If your doctor mentioned seeing a “lemon sign” or a “banana sign” during a fetal ultrasound, it is completely understandable to feel anxious. These are specific medical terms used to describe the shape of your baby’s skull and brain on an early ultrasound. They are classic markers that doctors look for to help diagnose open spinal dysraphism (the most common and severe form being myelomeningocele, or open spina bifida) ^[1]. While these terms describe structural changes in the head caused by the spinal opening, it is important to know that they are diagnostic tools. They help identify the condition, but they do not directly predict your baby’s future cognitive intelligence or IQ ^[2][3].

The Lemon Sign Explained

The lemon sign refers to an inward pulling or scalloping of the frontal bones of the baby’s skull ^[4]. Instead of having a typical round or oval shape, the front part of the skull looks slightly pinched, resembling a lemon ^[5].

This happens because an open neural tube defect causes cerebrospinal fluid (CSF) to leak out of the spine ^[6]. This loss of fluid creates lower pressure inside the skull, causing the flexible bones of the developing baby’s forehead to curve slightly inward ^[7]. The lemon sign is simply a physical reaction to the changes in fluid pressure, not a sign of brain damage. Notably, the lemon sign is primarily seen in the second trimester; as the baby’s skull bones harden and mature in the third trimester, the skull typically regains its normal, rounded shape, even though the spina bifida remains.

The Banana Sign Explained

The banana sign describes the appearance of the cerebellum, a part of the brain located at the back of the head that helps control balance and movement. Normally, the cerebellum looks like a dumbbell or figure-eight on an ultrasound. However, the same loss of spinal fluid pressure that causes the lemon sign also pulls the cerebellum downward ^[8].

As the cerebellum is pulled into the smaller space at the base of the skull, it becomes curved and flattened, taking on the shape of a banana ^[9]. This downward shift is known medically as Chiari II malformation ^[10]. While hearing the word “malformation” can be frightening, Chiari II is simply the medical name for this expected downward shift seen in almost all cases of open spina bifida.

What These Signs Mean for Your Baby’s Future

Hearing that your baby’s brain looks different can cause severe panic, but it is critical to separate the shape of the brain from how the brain will actually function.

• Not a Measure of Intelligence: Current medical research shows no direct link between the presence of the lemon or banana signs and your child’s future intelligence, learning abilities, or executive function ^[2][11].
• Focus on Function: Children with spina bifida can experience a range of cognitive profiles, but their development is influenced by a combination of factors over time. For instance, doctors will monitor your baby for hydrocephalus (a buildup of fluid in the brain) during future ultrasounds ^[12]. Hydrocephalus is a common, manageable condition that can be treated—often with a small tube called a shunt—if it develops.

What Happens Next?

Because the lemon and banana signs are caused by fluid leaking from the open spine, treating the spinal defect changes the pressure dynamics. For example, in-utero (prenatal) surgical repair of the spine often stops the fluid leak ^[13]. Research shows that prenatal repair can reverse the downward pull on the cerebellum (resolving the banana sign) and improve overall brain anatomy ^[14][15]. However, not every pregnancy is a candidate for fetal surgery, and surgery shortly after birth is also a highly effective, standard treatment option that safely manages the condition.

These ultrasound findings are just the first step in gathering information. Your care team will likely recommend further imaging, such as a fetal MRI, to get a clearer picture of your baby’s spine and brain to help guide your family’s treatment path.