Medical Genetics: How Chromosome Abnormalities Happen

Chromosomes are stick-shaped structures in the middle (nucleus) of each cell in the body. Each cell has 46 chromosomes grouped in 23 pairs. When a chromosome is abnormal, it can cause health problems in the body. Abnormal chromosomes most often happen as a result of an error during cell division. Chromosome abnormalities often happen due to 1 or more of these:

• Errors during dividing of sex cells (meiosis)

• Errors during dividing of other cells (mitosis)

Other causes of birth defects can include exposures of many types of substances (teratogens) to the developing baby.

Gendered terms are used here to talk about anatomy and health risk. Please use this information in a way that works best for you and your provider as you talk about your care.

Errors during dividing of sex cells (meiosis)

Meiosis (my-OH-sis) is the process in which sex cells divide and create new sex cells with half the number of chromosomes. Sperm and eggs are sex cells. Meiosis is the start of the process of how a baby grows. Normally, meiosis causes each parent to give 23 chromosomes to a pregnancy.  When a sperm fertilizes an egg, the union leads to a baby with 46 chromosomes.

But if meiosis doesn’t happen normally, a baby may have an extra chromosome (trisomy), or have a missing chromosome (monosomy). These problems can cause pregnancy loss or health problems in a child.

A woman age 35 years or older is at higher risk of having a baby with a chromosomal abnormality. This is because errors in meiosis may be more likely to happen as a result of the aging process. Women are born with all of their eggs already in their ovaries. The eggs begin to mature during puberty. If a woman is 35 years old, the eggs in the ovaries are also 35 years old. You may be referred for genetic counseling or testing if you’re age 35 or older when you are pregnant. Men make new sperm ongoing. So age doesn’t increase the risk for chromosome abnormalities for older fathers. But some newer studies suggest that rare abnormalities do occur.

Errors during dividing of other cells (mitosis)

Mitosis (my-TOH-sis) is the dividing of all other cells in the body. It’s how a baby in the womb grows. Mitosis causes the number of chromosomes to double to 92, and then split in half back to 46. This process repeats constantly in the cells as the baby grows. Mitosis continues throughout your lifetime. It replaces skin cells, blood cells, and other types of cells that are damaged or naturally die.

During pregnancy, an error in mitosis can occur. If the chromosomes don’t split into equal halves, the new cells can have an extra chromosome (47 total) or have a missing chromosome (45 total).      

Substances that cause birth defects (teratogens)

A teratogen (ter-AT-uh-jen) is something that can cause or raise the risk for a birth defect in a baby. They are things that a birth parent may be exposed to during pregnancy. Teratogens include:

• Some medicines

• Street drugs

• Alcohol

• Tobacco

• Toxic chemicals

• Some viruses and bacteria

• Some kinds of radiation

• Certain health conditions, such as uncontrolled diabetes

Researchers have 2 ways of finding out if a substance is a teratogen:

• Animal studies. Animal studies are the main way to find out if a medicine or other substance is safe during human pregnancy.

• Observations from human exposure. These are the reports of problems about a substance over time.