Aneuploidy

Aneuploidy is a genetic disorder where the total number of chromosomes doesn’t equal 46. If there’s an extra chromosome copy (trisomy) you’ll have 47. If you’re missing a chromosome copy (monosomy), you’ll have 45. Any change in the number of chromosomes could affect the outcome of a pregnancy.

What are chromosomes?
Humans have chromosomes, which are thread-like structures inside the nucleus of each cell. Chromosomes carry DNA that surround proteins like a tight spring. DNA is your body’s instruction manual that your biological parents pass down to you. Your DNA is what makes you unique. You’ll receive 23 chromosomes from each of your biological parents (46 total). These chromosomes are arranged into pairs; we inherit one chromosome of each numbered pair of chromosomes (1 through 22) from each parent; and then genetic males inherit and X and a Y, while genetic females inherit two X chromosomes.

Cells have a short lifespan and they constantly need to be replaced. Cell division happens when new cells replace old cells, similar to how you would use “copy and paste” on your computer. During this process, DNA distributes evenly throughout new cells when the chromosome pairs in the old cell separate from each other, then copy themselves. This happens throughout our lives but also in the creation of egg and sperm cells. Sometimes, chromosomes aren’t divided equally, causing the number of a specific chromosome someone has to be inaccurate. This can lead to a genetic condition diagnosis like Turner syndrome or Down syndrome.

What is aneuploidy?
Aneuploidy occurs when the number of chromosomes a cell has doesn’t equal 46. While this can happen sometime during your lifetime, more often, it occurs when the number of chromosomes your child acquires from their biological parents doesn’t equal 46 due to an error in the creation of the egg or sperm. There can be an extra copy of a chromosome (trisomy) or a missing copy of a chromosome (monosomy), making the total number of chromosomes not a multiple of 23.

Changes in the number of chromosomes from each biological parent can affect the outcome of a pregnancy, most often resulting in a miscarriage. Studies show that aneuploidy affects almost half of pregnancies that end in a miscarriage in the first trimester.

What are the types of aneuploidy?
There are two main types of aneuploidy; there can be an extra copy of a chromosome (trisomy) or a missing copy of a chromosome (monosomy). Humans have 23 pairs of chromosomes, totaling 46 chromosomes. A person with an aneuploidy condition typically has either one more or one less chromosome than normal.

Trisomy
Trisomy occurs when a person has an extra copy of a chromosome. A baby with trisomy has 47 chromosomes. Common conditions that are the result of trisomy include:

Down syndrome: An extra copy of chromosome 21.
Trisomy 18: An extra copy of chromosome 18 that used to be called Edwards syndrome.
Trisomy 13: An extra copy of chromosome 13 that used to be called Patau syndrome.
Monosomy
Monosomy occurs when a person is missing a copy of a chromosome. Their total chromosome count equals 45. A common condition that’s the result of monosomy is Turner syndrome.

A sex chromosome determines your baby’s sex identity assigned at birth. There are two sex chromosomes, X and Y (XX for female and XY for male). A Turner syndrome diagnosis, or monosomy X, occurs when your baby only has one X sex chromosome instead of a pair.

Who does aneuploidy affect?
Fetal aneuploidy can affect any baby. The risk of having a child with fetal aneuploidy is higher among individuals who have a higher maternal age (with the exception of Turner syndrome). For example, at 20 years of age, your risk of having a child with a chromosome abnormality is 1 out of every 1,480 pregnancies. At age 40, your risk is higher, estimating 1 out of every 65 pregnancies for having a child with a chromosome abnormality.

If you’re planning on becoming pregnant, talk with a healthcare provider about genetic counseling to screen for potential genetic conditions.


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