Prenatal ultrasound is a common part of pregnancy care, providing early insights into fetal health and development. This non-invasive imaging technique allows healthcare providers to monitor growth and identify potential issues. It is a primary tool for detecting neural tube defects.
What Are Neural Tube Defects?
Neural tube defects (NTDs) are birth defects affecting the brain, spine, or spinal cord. These conditions arise early in pregnancy, when the neural tube—the embryonic structure that forms the brain and spinal cord—does not close completely. This incomplete closure can lead to neurological damage and physical challenges.
Spina bifida and anencephaly are common types of NTDs. Spina bifida occurs when the spinal column does not form properly, affecting the spinal cord and nerves. Myelomeningocele is the most serious type, where the spinal cord and its protective coverings protrude through an opening in the back. Anencephaly, a more severe defect, involves the incomplete development or absence of major parts of the brain, skull, and scalp. Infants with anencephaly are usually stillborn or survive only for a short time.
Ultrasound’s Role in Detection
A detailed fetal anatomy ultrasound, often performed between 18 and 22 weeks of gestation, is a primary method for detecting neural tube defects. This scan, sometimes referred to as a Level II ultrasound or anomaly scan, uses sound waves to create images of the fetus, placenta, and amniotic fluid. A computer translates the echoes into real-time images displayed on a monitor.
During this scan, sonographers and doctors look for specific visual markers that indicate an NTD. One marker is the “lemon sign,” an indentation of the frontal bones of the skull. Another is the “banana cerebellum,” where the cerebellum appears compressed and elongated. Direct visualization of spinal defects is also sought, such as an open spine, bony abnormalities, or a sac-like protrusion along the backbone. These images allow for assessment of the fetal brain, spine, and other anatomical structures.
Evaluating Ultrasound Accuracy
While ultrasound is an effective tool for detecting neural tube defects, its accuracy is not absolute. Several factors can influence its ability to identify these conditions. The gestational age at which the scan is performed plays a role, with the detailed anatomy scan in the second trimester generally offering the best visualization.
The position of the fetus within the uterus can obscure views, making detection challenging. Maternal body habitus can also affect image quality. The skill and experience of the sonographer and physician interpreting the images are important determinants of accuracy. While ultrasound is a powerful screening tool, the possibility of false negatives (undetected defects) or false positives exists.
Navigating a Suspected Diagnosis
If an ultrasound suggests the presence of a neural tube defect, further diagnostic steps are recommended. This often involves additional, more specialized ultrasounds to confirm initial findings and gather more detailed information about the defect. Another common diagnostic test is amniocentesis, a procedure where a small sample of amniotic fluid is collected.
This fluid can be tested for elevated levels of alpha-fetoprotein (AFP) and acetylcholinesterase, which are biochemical markers often associated with open neural tube defects. Genetic counseling is also an important step, providing families with information about the condition, recurrence risks, and available support resources. Referral to specialists, such as pediatric neurosurgeons or maternal-fetal medicine experts, helps ensure a comprehensive understanding and management plan for the pregnancy and beyond.