Chromosomal Abnormalities and Pregnancy Loss

Understanding the Link Between Genetic Changes and Pregnancy Loss

While many miscarriages and stillbirths happen for unknown reasons, when we do know the cause, it is most often a chromosomal abnormality. The American College of Obstetricians and Gynecologists (ACOG) estimates that as many as 50% of first-trimester miscarriages occur due to chromosomal abnormalities. Read more about why chromosomal abnormalities can cause miscarriage.

What is a Chromosome?

Chromosomes are tiny, thread-like pieces of genetic material (genes) that carry all of the information that makes each person unique. Human beings normally have 23 pairs of chromosomes for a total of 46. Half (23) come from the father in sperm and half from the mother in her egg.

What Causes Abnormalities?

No one knows exactly what causes a sperm or egg to develop abnormally. We know there can be tiny changes in a single gene (as in translocation, or deletion), or much larger changes, such as an entire chromosome missing or having an extra copy of a chromosome. When a whole chromosome is missing or duplicated, this is called aneuploidy. There can also be changes shortly after fertilization (when a sperm and egg are joined) where not all cells have the same genetic information. This is called mosaicism.

Which Chromosomal Abnormalities Cause Miscarriage?

This list is by no means exhaustive, as we may never know every change in a chromosome that occurred in a miscarriage. Many miscarriages occur before a woman even knows she is pregnant, and in early miscarriage -- especially first losses -- many women opt not to have further testing. However, there are a number of abnormalities that are known to be “incompatible with life” and usually result in pregnancy loss.

Aneuploidy

As previously mentioned, aneuploidy refers to a change in an entire chromosome. While there can be three or more complete sets of 23 chromosomes (also known as triploidy), it is more common to have only one chromosome affected. Not all aneuploidies are associated with miscarriage. The most well-known is Down syndrome, where chromosome 21 has an extra copy, for a total of 47 chromosomes in affected individuals (this is referred to as trisomy). Others, such as Turner syndrome or Triple X syndrome, are not life-threatening. Aneuploidy in chromosomes 8, 9, 13, 15, 16, 17, 18, and 22 are associated with miscarriage.

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Translocation

When a small piece of a chromosome is out of place, it is called a translocation. In ​a balanced translocation, small pieces of two different chromosomes have traded places -- for example, if small pieces of chromosomes 12 and 22 “traded places.” In unbalanced translocation, a small piece of one chromosome is attached to a different chromosome without any missing pieces. People with balanced translocations usually live normal lives and do not know they have a genetic abnormality. However, they have a 50% chance of producing sperm or eggs with an unbalanced translocation, which can lead to miscarriage.

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Testing for Chromosomal Abnormalities

It is often possible to get chromosome testing done immediately after a miscarriage. Most doctors recommend genetic testing for women who have recurrent miscarriages, but anyone looking for a definitive cause for a pregnancy loss can inquire about testing.

It is also possible to get genetic testing done during a pregnancy. For more information about testing, please see the following:

Sources:

March of Dimes. Chromosomal Abnormalities. Accessed 4 Dec 2011.

American College of Obstetricians and Gynecologists. FAQ: Genetic Disorders. Accessed 4 Dec 2011.

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