We hear the words “congenital” and “heredity” all the time, sometimes even in a nonmedical sense. Someone can be called a congenital liar, for example. We also talk about heredity nonmedically, too, like when someone inherits a fortune. In medicine, birth defects can be described as congenital or inherited, and there is overlap between the two, but also an important difference.
A birth defect is any malformation or condition that is present from when the baby is born. Congenital means “from birth.” Birth defects can range from the very minor—like a red blotch on the baby’s skin that disappears in a few weeks—to extremely severe, such as a missing limb or a defect of the heart or brain. They also include problems that are present from birth, but which may not be visible at birth, such as a condition like sickle cell anemia or cystic fibrosis, which sometimes are not diagnosed until years later.
According to the Cleveland Clinic, 2% to 3% of all babies show a significant defect at birth. The cause of a birth defect can be identified in only about 30% of all birth defects, which means in seven out of every ten cases, a definite cause is never found.
There are two basic types of birth defects, according to the Eunice K. Shriver National Institute of Child Health and Human Development. A problem with the shape or formation of a structure, like an arm or a leg, is called a structural disorder. A problem with how well a body part like the heart or intestinal system works is called a functional disorder. Some birth defects fall into both categories.
A congenital problem that is not due to genetics can be caused by many different things. Exposure to certain chemicals during pregnancy can cause a birth defect, as can some drugs, smoking, or excessive alcohol use. Not getting enough of the nutrient folate during pregnancy has been shown to cause problems with development of the spine and brain.
An infection contracted during pregnancy can cause a congenital defect. Rubella (German measles) is known to cause birth defects that can vary depending on when during pregnancy the mother got sick. According to the Cleveland Clinic, “Rubella causes cataracts if infection occurs during the sixth week of pregnancy, deafness if the infection occurs during the ninth week, and heart defects if the infection occurs between the fifth and tenth week of pregnancy.”
Genetic versus Inherited
Inherited means that the condition was passed to the baby by its parents in his or her genes. Genes are units of information that are coded into the chromosomes, the strings of genes needed to build and maintain a living creature. An inherited condition can be caused by genes that either (or both) parents carry that they pass on to the baby, including such conditions as Tay-Sachs disease or phenylketonuria. With conditions like cystic fibrosis, both parents have to be a carrier of the disease and the baby has to inherit two copies of the gene, one from each parent. With other inherited diseases, only one gene needs to be inherited. For some diseases, several genes must be inherited.
What makes the “congenital versus inherited” issue more confusing is that some diseases are caused by genes but are not inherited from the parents. A gene (or several genes) may mutate in the egg or sperm as it forms the fertilized egg that then the embryo.
Humans have 23 pairs of chromosomes. Some genetic conditions are caused when a baby gets three copies of a chromosome or an extra portion of a chromosome during the creation of the embryo. These are called chromosomal disorders. For example, Down syndrome occurs when a baby inherits three copies of chromosome 21. Heredity is not a factor in nearly all cases of Down syndrome and other chromosomal disorders, since the extra chromosome happened at fertilization. However, there is a rare type of Down syndrome called translocational Down syndrome where the baby has inherited only 23 pairs of chromosomes but has an additional copy or partial copy of chromosome 21 attached to another chromosome. This rare type may be inherited from a parent.
Whether a birth defect is inherited from a parent rather than being due to some other cause is important if or when the parents are considering having more children. According to the Cleveland Clinic, about 20% of birth defects are caused by genetic or inherited factors.