Spina bifida (SB) is a common neural tube defect occurring when the structure forming the baby’s brain and spinal cord, the neural tube, does not completely close during early development. This results in a gap in the spine where the bones, or vertebrae, have not properly formed around the spinal cord. Prenatal ultrasound is the primary, non-invasive method used to detect this condition. Sonographers look for a combination of direct physical signs on the spine and indirect changes that appear in the fetal brain.
The Timing and Purpose of Prenatal Screening
The detection of spina bifida typically occurs during the mid-pregnancy anatomy scan, also known as the Level II ultrasound. This detailed screening is usually performed between 18 and 22 weeks of gestation. This timing is important because the fetal structures are large enough to be clearly visualized, while still providing a window for additional testing and management decisions, including potential fetal surgery.
The Level II ultrasound is highly effective for detecting open forms of spina bifida, which involve an exposed spinal cord. It is less successful at identifying milder, closed forms, such as spina bifida occulta. In the closed form, the bony defect is small and covered by skin, often making it difficult to see on a standard two-dimensional ultrasound. Early detection in the second trimester is a major goal for medical teams.
Direct Visual Markers on the Fetal Spine
The most direct sign of spina bifida on ultrasound is the visible defect in the fetal spine itself. When viewed in cross-section, normal vertebral bodies appear as a closed circle or protective ring around the spinal cord. In a fetus with open spina bifida, this bony ring is incomplete, displaying a characteristic “U” or “V” shape instead of normal parallel lines.
This abnormality is known as an open posterior neural arch, indicating that the bony elements have splayed apart. At the defect site, the ultrasound may show a fluid-filled sac protruding from the back. This sac is called a myelomeningocele, and it contains the exposed spinal cord, nerves, and surrounding membranes. The size and exact location of this cystic mass are carefully documented, as the level of the defect relates to the condition’s potential severity.
Indirect Visual Markers in the Fetal Brain
The most reliable and easiest-to-spot signs of spina bifida are often found in the fetal head, not the spine. The spinal defect allows cerebrospinal fluid to leak out, creating a pressure imbalance that pulls the brain structures downward. This downward traction of the brain tissue is known as Chiari Malformation Type II.
The Lemon Sign
The resulting abnormal head shape is known as the “Lemon Sign.” This term describes the scalloping or flattening of the frontal bones, which makes the fetal skull appear indented, similar to the shape of a lemon. This sign is an indicator of low intracranial pressure and is highly associated with spina bifida in the second trimester. It is present in nearly all affected fetuses before 24 weeks.
The Banana Sign
Another indirect sign is the “Banana Sign,” which is also caused by Chiari Malformation Type II. The cerebellum, located at the back of the brain, is pulled down into the spinal canal, causing it to lose its normal, rounded shape. On an axial view, the compressed cerebellum appears flattened and curved around the brainstem, resembling a banana. This sign is considered a highly consistent feature of open spina bifida, present in over 95% of cases.
Ventriculomegaly
These cranial changes are accompanied by ventriculomegaly, which is the enlargement of the fluid-filled spaces in the brain. This occurs because the downward displacement of the brainstem obstructs the normal flow of cerebrospinal fluid. Although the lemon sign may disappear as the pregnancy progresses, the banana sign and ventriculomegaly remain important markers for diagnosis.
Confirming the Diagnosis and Follow-Up Imaging
If an ultrasound suggests spina bifida, specialized testing is performed to confirm the diagnosis and assess the defect’s full extent. A high-resolution fetal Magnetic Resonance Imaging (MRI) scan often supplements the ultrasound. While ultrasound is excellent for initial screening, fetal MRI provides greater detail about the spinal cord, nerve roots, and associated brain changes, especially in later stages of pregnancy.
Maternal blood tests for alpha-fetoprotein (MSAFP) are also used in screening. Abnormally high MSAFP levels suggest an open neural tube defect, as the exposed fetal tissue leaks this protein into the maternal bloodstream. If the diagnosis is confirmed, the expectant parents are referred to a multidisciplinary team, including maternal-fetal medicine specialists and neurosurgeons, to discuss management options.