The majority of pregnancies result in healthy babies. More often than not, when a woman finds out she is pregnant she will go on to have a healthy pregnancy with few complications. One of the purposes of ultrasound during pregnancy is to detect any abnormalities in the growing baby. At the 20-week ultrasound, the baby is big enough to look at the full anatomy of the baby- we can analyze the health of the heart, kidney, lungs, brain, etc. While ultrasound cannot rule out all health problems, ultrasound can typically detect major birth defects. Approximately 3-5% of all pregnancies can develop major birth defects. In particular, one such birth defect that is screened for in pregnancy are neural tube defects (NTD). NTDs occur in approximately 1/1000 births worldwide.
How Neural Tube Defects Occur
At conception, an egg and sperm combine to form an embryo. As an embryo grows, cells copy their genetic instructions and divide over and over. Early on in the development of a fetus, a layer of cells in the fetus form a tube called the neural tube. The neural tube will later form the spinal cord, the brain, and structures nearby that protect both the spinal cord and brain. As the neural tube forms, it is important that the tube completely “zips up” or closes to form a complete tube. The top of the tube will form the brain and the remainder forms the spinal cord. A NTD occurs when the tube does not completely “zip up” or close, typically at the top or bottom of the tube, but sometimes in the middle of the tube as well.
When the top of the neural tube fails to close, it can result in a baby diagnosed with anencephaly. Anencephaly is a less common but more severe type of NTD that results in most or all of the brain missing, as well as parts of the skull. Infants born with anencephaly typically remain unconscious and die soon after birth.
If the bottom or other areas of the neural tube fails to close completely, it can result in spina bifida, the most common type of NTD. An infant born with spina bifida will have a bulge in between parts of the spine. This bulge can often contain the spinal cord and the cerebral spinal fluid. Depending on the severity of the bulge, infants can have symptoms anywhere from completely mild with no disability to problems walking, intellectual disabilities, and difficulties with bladder and bowel functions.
While NTDs often occur sporadically, there are some factors that we know can play a role. Maternal nutrition (such as alcohol and caffeine use, low dietary quality, etc.) can increase the risk for NTDs in pregnancy. In addition, it has been thoroughly documented since the 1950s that taking folic acid during pregnancy reduces the risk of NTDs. If a woman is planning on becoming pregnant, it is recommended to start taking a folic acid supplement of about 400mcg prior to conception. Other environmental factors that could increase the risk of NTDs in pregnancy include air pollution, pesticides, and nitrate-related compounds.
The Genetic Component of NTDs
From a genetic perspective, fewer than 10% of neural tube defects are associated with a genetic disorder. The most common chromosome abnormalities associated with NTDs, which all women have a risk to be diagnosed with in pregnancy, are Trisomy 18, Trisomy 13, and triploidy. These chromosome abnormalities occur sporadically in the majority of cases. Another syndrome called Meckel syndrome, which includes brain malformations, kidney abnormalities, and extra digits, is associated with a type of NTD called encephalocele (a bulge at the base of the neck).
If you are diagnosed with an NTD during pregnancy, it is important to discuss your options with your healthcare provider or a genetic counselor. In addition, a genetic counselor can determine your risk to have a future pregnancy diagnosed with an NTD. Depending on your family history and personal medical history, the risk of a future pregnancy being diagnosed with an NTD can be up to 3%.
If you would like to discuss your options or your family history related to NTDs, please contact AT-GC to meet with a genetic counselor.