About 1 in 150 babies in the US is born with a chromosomal condition. Babies with chromosomal conditions have a problem in one or more of their chromosomes. Chromosomes are the structures that hold genes. Genes are part of your body’s cells that store instructions for the way your body grows and works. Genes are passed from parents to children.
Each person has 23 pairs of chromosomes, 46 in all. For each pair, you get one chromosome from your mother and one chromosome from your father.
Chromosomal conditions are caused by two kinds of changes in chromosomes:
1. Changes in the number of chromosomes—This means there are too many or too few chromosomes.
2. Changes in the structure of chromosomes—This means that part of a chromosome may be missing, repeated or rearranged.
Chromosomal conditions usually result from an error that occurs when an egg or sperm cell develops. It is not known why these errors occur and as far as we know, nothing that a parent does or doesn’t do before or during pregnancy can cause a chromosomal abnormality in their child. In some cases these kinds of changes can be inherited or passed from parent to child.
In most cases, an embryo with the wrong number of chromosomes does not survive. In such cases, the pregnant woman has a miscarriage. This often happens very early in pregnancy, before a woman may realize she’s pregnant. More than 50 percent of first-trimester miscarriages are caused by chromosomal abnormalities in the embryo.
The American College of Obstetricians and Gynecologists recommends that all pregnant women be offered a screening test for Down syndrome and certain other chromosomal abnormalities. Screening may consist of a maternal blood test done in the first trimester (at 11 to 13 weeks of pregnancy) along with a special ultrasound of the back of the baby’s neck. It also can be a maternal blood test done in the second trimester (at 15 to 20 weeks). A screening test helps identify pregnancies that are at higher-than-average risk of these conditions.
However, a screening test cannot diagnose Down syndrome or other chromosomal abnormalities. If a screening test shows that your baby may have a problem, your provider gives you a diagnostic test that can tell you if your baby actually does or does not have a chromosomal condition.
Each child born with a chromosomal condition is different. Some children with chromosomal conditions have intellectual disabilities or birth defects, or both. But some children with these conditions don’t have any serious problems. The problems depend on which chromosomes are affected and how they are affected.
If you or someone in your family has a chromosomal condition, or if you have a baby with a chromosomal condition, talk to a genetic counselor. A genetic counselor can help you understand the causes of chromosomal conditions, what kind of testing is available, and your chances of having a baby with these conditions.