Shortly after conception, a fetus' ventricular system begins to develop. It starts as a hollow tube that fills with cerebrospinal fluid, or CSF, as the fetus grows. This tube eventually separates from the amnion, the sac in which your baby is growing, and forms the choroid plexuses -- or ventricular system of the brain. Ultimately, there will be four ventricles, and by your second trimester, each of them will be filled with a constant ebb and flow of CSF.
Once your baby's ventricular system is up and running, her brain takes over the production of cerebrospinal fluid which moves through the ventricles into reservoirs located at the base of the brain, according to the National Institute of Neurological Disorders and Stroke. From here, CSF cushions her developing brain and protects it against harm from physical blows to your abdomen or other jarring. CSF also carries nutrients to your baby's brain for healthy growth before her blood reabsorbs it. One pair of these ventricles, called the lateral ventricles and located at the very center of your baby's brain, should ideally measure half an inch, or 10 mm each.
Complications can occur with your baby's development when too much CSF gathers in his ventricles and widens them, according to the National Institute of Neurological Disorders and Stroke.This widening, or ventriculomegaly, usually occurs in the lateral ventricles. Mild ventriculomegaly is a widening from the normal 10 mm up to 15 mm and affects approximately one in every 1,000 babies, according to Kaiser Permanente. A widening greater than 15 mm is severe. Your baby's brain continuously produces CSF, so anything that prevents it from moving through the ventricles and reabsorbing can potentially cause problems, such as hydrocephalus -- in laymen's terms, fluid on the brain. Hydrocephalus is usually the result of severe ventriculomegaly.
It can be difficult to pinpoint the cause of ventriculomegaly, although infections such as cytomegalovirus and toxoplasmosis have been implicated. In some cases, the ventricles are congenitally defective, preventing the normal flow of CSF, according to the website Obfocus.
Your obstetrician will monitor your baby's ventricles as a matter of course beginning in your second trimester, usually by ultrasound. If she detects a problem, she might offer special testing, such as a high-resolution ultrasound, a fetal MRI or amniocentesis to check for chromosome abnormalities.
The wider your baby's ventricles, the more potential there becomes for disabilities, according to Kaiser Permanente. Babies with ventriculomegaly can develop birth defects, including heart and spinal abnormalities. Approximately 4 percent of these infants will exhibit chromosome abnormalities -- but more than 95 percent will not. The most common chromosomal abnormality linked to ventriculomegaly is Down's Syndrome. Most babies with mild ventriculomegaly -- approximately 77 percent -- show no problems with learning or development later, especially with ventricular measurements under 12 mm. Ventricular size may begin to decrease as your pregnancy progresses. In only about 8 to 10 percent of these babies will ventricular size increase over the course of gestation.