Conditions of the fetal central nervous system (CNS) are among the most frequently diagnosed congenital abnormalities, occurring at a rate of about 10 per 1,000 live births. Thanks to advances in sonographic imaging, care providers are able to examine the fetal CNS in detail during a prenatal sonogram, which is usually performed when the fetus is between 19 and 22 weeks old. (However, severe congenital abnormalities, such as anencephaly and holoprosencephaly, can be diagnosed as early as 11 to 13 weeks.) The detection rate for CNS in the first trimester is nearly 70 percent.
The sonogram allows for identification of abnormalities in the CNS, which then lets care providers classify the conditions based on the ultrasound results. After identifying abnormalities and determining the severity of the condition, providers will provide a diagnosis and develop a plan for treatment. The earlier the abnormalities are detected, the more time care providers and parents have to plan for the outcome of the pregnancy and for postnatal management of conditions.
What Are Fetal Central Nervous System Conditions
The central nervous system is composed of the brain and the spinal cord, and both are surrounded by cerebrospinal fluid, which is protected by a thin lining (meninges). The spinal cord attaches to the brain stem at the base of the brain and runs through the vertebral column to the lumbar region in the lower spine. The spinal cord transmits nerve signals from the brain to the rest of the body and coordinates many of the body’s reflexes.
In a fetus, the CNS starts growing as a simple structure known as the neural groove, which then forms a neural tube that becomes the brain and spinal cord. Typically, the neural tube will close and fuse by the 28th day after conception, and when that doesn’t happen, it results in a birth defect of the CNS known as a neural tube defect. It can result in a number of different fetal CNS conditions, with two of the most common conditions being spina bifida and microcephaly.
What Causes Fetal Central Nervous System Conditions
Several factors can cause abnormalities in the fetal CNS. In some cases, it may be caused by genetic factors, although the specific connection between genetics and CNS conditions is unclear. However, a woman who has a family history of neural tube defects — such as a relative who has given birth to a baby with this condition or who has already given birth to a baby herself who had CNS abnormalities — is at a higher risk of having babies with a similar condition.
However, environmental factors also play a role in CNS abnormalities. Among the factors that can contribute to neural tube defects are:
- Folic acid deficit. Folic acid is critical to produce red blood cells and helps the baby’s neural tube develop into a brain and spinal cord. Since birth defects occur within the first three or four weeks of pregnancy, it’s important for mothers-to-be to have folate in their system before getting pregnant and particularly during the early stages of pregnancy.
- Obesity. In addition to experiencing other pregnancy complications, women who are obese when they become pregnant have an increased risk of having a baby with fetal CNS disorders.
- Use of certain medication. Women taking medication to control seizures, such as medication for epilepsy, have a higher rate of births to babies with CNS abnormalities.
- Poorly controlled diabetes. Mothers with diabetes (either Type 1 or Type 2) at the early stages of pregnancy who have abnormally high glucose levels are at greater risk for having babies with neural tube disorders.
In some cases, no specific cause for the condition can be identified.
Detection of Fetal Central Nervous System Conditions
Abnormalities in the fetal CNS are usually diagnosed during pregnancy, either through imaging tests such as ultrasound, which is usually effective in detecting nearly every type of CNS condition, or through lab tests such as a triple screen blood test. This blood test is usually done in the second trimester and looks for elevated levels of AFP (alpha-fetoprotein), which are associated with neural tube defects. The test also screens for estriol and human chorionic gonadotropin (hCG). Amniotic fluid may also be tested for high AFP levels and for abnormalities in the chromosomes.
When a baby has been diagnosed with or is suspected to have a condition of the CNS, he or she will be tested shortly after birth to determine the severity of the defect and screen for any complications. Some of the tests performed may include:
- Magnetic resonance imaging (MRI)
- Computed tomography (CT or CAT scan)
- Measurement of the infant’s head circumference
- Assessments of the baby’s strength, motor skills and sensory functioning
What Treatment Options Are Available for Fetal Central Nervous System Conditions
Since abnormalities in the fetal CNS occur very early in the baby’s development, the baby’s nerve damage and loss of function are almost always permanent. However, there are treatments available for individual conditions that can sometimes help prevent further damage from occurring and can help with any complications that arise.