Obstetrics

Diagnosis & Imaging: Understanding Your Ultrasound and MRI Results

At a Glance

Neural tube defects are primarily diagnosed using high-resolution ultrasounds and fetal MRIs to examine the baby's brain and spine. Key imaging details, such as the lesion level and the presence of a Chiari II malformation, help doctors determine if a baby is a candidate for fetal surgery.

Diagnostic imaging acts as a window, allowing doctors to understand the unique details of your baby’s development. While a screening blood test often starts the process, high-resolution ultrasound and Fetal MRI provide the detailed “map” needed to plan for your baby’s care ^[1]^[2].

The Diagnostic Path

The journey toward a diagnosis typically follows three main steps:

MS-AFP (Maternal Serum Alpha-Fetoprotein): This is a simple blood test performed in the second trimester ^[1]. AFP is a protein made by the baby; if the neural tube is “open,” higher amounts of this protein leak into the mother’s blood ^[2]. While it is a helpful screening tool, it is not a final diagnosis ^[1].
High-Resolution Ultrasound: This is the primary tool for diagnosing neural tube defects ^[2]. Specialists use it to look at the spine, the brain, and the baby’s movements in real-time ^[3].
Fetal MRI: If a defect is found on ultrasound, a Fetal MRI may be ordered ^[4]. This provides highly detailed images of the brain and spinal cord tissue, helping doctors see things that ultrasound might miss, such as the specific details of a Chiari II malformation ^[5]^[6].

Understanding the Imaging Report

When you read an imaging report, you may see several specialized terms used to describe how the brain and spine are shaped.

Lesion Level: This identifies the exact vertebra where the opening in the spine begins (e.g., L3 or S1) ^[7]. It is the most important predictor of future mobility ^[8].
Ventriculomegaly: This refers to the enlargement of the fluid-filled spaces (ventricles) in the brain ^[9]. It occurs when the flow of cerebrospinal fluid is disrupted, a condition often called hydrocephalus, which is a fluid buildup that puts pressure on the brain ^[10].
Chiari II Malformation: Also called “hindbrain herniation,” this is when the lower part of the brain (the cerebellum and brainstem) is pulled downward toward the spinal canal ^[11]^[12].
The ‘Lemon’ Sign: A shape seen on ultrasound where the front of the skull looks slightly indented, resembling a lemon ^[3].
The ‘Banana’ Sign: A term used when the cerebellum is pulled tight and curved around the brainstem due to the Chiari II malformation, giving it a banana-like shape ^[13].

Completeness Checklist for Families

To make informed decisions about treatments—including whether you might be a candidate for fetal surgery—your medical team needs a “complete map” from the imaging ^[14]. Ensure your report or doctor has addressed the following:

[ ] Type of Defect: Is it “open” (myelomeningocele) or “closed” (meningocele)? ^[15]
[ ] Highest Lesion Level: What is the specific vertebra where the defect starts? ^[7]
[ ] Brain Anatomy: Is a Chiari II malformation present? ^[14]
[ ] Ventricular Size: What is the measurement (in millimeters) of the brain’s ventricles? (Measurements over 15mm may change treatment options) ^[16]
[ ] Associated Findings: Are there other features like clubfoot (feet turned inward) or scoliosis (curvature of the spine)? ^[8]
[ ] Fetal Movement: Did the baby move their legs and feet during the ultrasound? ^[17]

Common questions in this guide

How is a neural tube defect diagnosed during pregnancy?

Diagnosis typically begins with an MS-AFP maternal blood test to check for elevated protein levels. If the screening is positive, specialists use a high-resolution ultrasound to examine the baby's spine and brain, often followed by a fetal MRI for highly detailed tissue imaging.

What does the lesion level mean on my baby's ultrasound report?

The lesion level identifies the exact vertebra in the spine where the opening begins, such as L3 or S1. This measurement is a critical predictor of your baby's future mobility and helps doctors evaluate if fetal surgery is an option.

What is a Chiari II malformation?

A Chiari II malformation, sometimes called hindbrain herniation, happens when the lower part of the baby's brain is pulled downward toward the spinal canal. It is commonly identified on fetal imaging in babies with open neural tube defects.

What do the lemon and banana signs mean on a fetal ultrasound?

These terms describe the distinct shape of the baby's head and brain on an ultrasound. The lemon sign refers to a slightly indented skull, while the banana sign describes a curved cerebellum caused by a Chiari II malformation pulling the brain downward.

How do imaging results affect fetal surgery options?

To qualify for prenatal in-utero surgery, the medical team needs a complete imaging map. They review the specific lesion level, the size of the brain's ventricles, and overall brain anatomy to determine if fetal surgery is a safe and appropriate treatment.

Get notified when new evidence is published on Neural tube defect.

We monitor PubMed for new peer-reviewed studies on this topic and email a short summary when something meaningful changes.

References (17)

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This page provides educational information about fetal imaging for neural tube defects. Always review your specific ultrasound and MRI reports with your maternal-fetal medicine specialist or obstetrician.

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