Down syndrome is the most commonly occurring chromosomal abnormality during pregnancy. It occurs when there is an extra copy of chromosome 21. This can cause mental impairment and physical abnormalities. Although there is no way of preventing Down syndrome, there are signs in pregnancy that can determine if a baby has it.
According to the Down Syndrome Society, Down syndrome occurs in 1 out of 733 pregnancies. As a woman gets older, her risk for having a child with the condition increases dramatically. A woman who is 45 years old has a 1 in 30 chance of having a child with Down syndrome.
During the 1st trimester, many women have their first ultrasound. One of the reasons is to look for physical abnormalities. One of the physical abnormalities that a fetus may have is nuchal translucency. This is where the skin of the neck appears thickened. Fluid collects in this area when a fetus has Down syndrome. An enlarged skin fold in the neck is a sign of Down syndrome.
There are several other physical abnormalities that may be seen on an ultrasound examination that may be an indicator of Down syndrome. These are called “soft markers” because they do not necessarily mean that the baby has the condition. Further testing would be required in order to make a determination. Lack of a nasal bone, dilation in the kidneys, and bowel or heart defects can all be signs that Down syndrome may be present.
Abnormal Blood Tests
Another sign of Down syndrome during pregnancy is an abnormal blood test. During the first trimester, blood testing is done that measures a set of biochemical markers. An abnormal measurement of pregnancy-associated plasma protein A (PAPP-A), or human chorionic gonadotropin (HCG) could be a sign of Down syndrome.
In the second trimester, a blood test called the quad screening is done. This blood test measures alpha-fetoprotein, estriol, HCG, and inhibin-A. Abnormal results on the quad screening could indicate Down syndrome. False positives are common in blood tests, so your doctor will order more tests to help confirm and make a diagnosis.
If any of the signs that the baby may have Down syndrome are present, your doctor will confirm the diagnosis using more invasive tests. Amniocentesis is a test whereby a long needle is inserted into the uterus and cells are taken from the amniotic fluid. Chorionic villus sampling is a test that involves taking cells from the placenta, and percutaneous umbilical blood sampling is a test in which blood is taken from the umbilical cord. All three of these tests confirm Down syndrome by the presence of an extra chromosome. According to the Mayo Clinic, these tests are 98 percent accurate in confirming Down syndrome. They all carry a small risk of miscarriage, so are usually only performed if other signs of Down syndrome are present, or if the mother is at risk or over age 35.