Your Guide to Chromosomal Abnormality and Pregnancy

How Chromosomal Abnormality Can Cause Miscarriage, Down Syndrome, and More

Mother playing with baby girl with Down Syndrome
Mother playing with baby girl with Down Syndrome. Getty Images/JGI/Tom Grill

A chromosomal abnormality occurs when a fetus has an incorrect number of chromosomes or has chromosomes that contain structural abnormalities (such as missing or extra pieces).

What Are Chromosomes?

Since you might not have taken a biology course recently, here's a quick refresher on chromosomes. Your body is made up of cells. In the middle of each cell, there is a nucleus. Inside that nucleus, you'll find chromosomes.

Chromosomes are important because they contain genes, which are made up of DNA. Genes are what determine how you look, what blood type you have, how susceptible you are to disease, and more.

Your Body's Chromosomes

Each cell in the body usually contains 46 chromosomes—23 pairs that hold roughly 20,000 to 25,000 genes, according to the National Human Genome Research Institute. Half of your chromosomes come from the egg from your mother and half come from the sperm from your father.

Of the 23 pairs of chromosomes, the first 22 pairs are called "autosomes." The final two chromosomes are called "allosomes" or sex chromosomes. Allosomes determine an individual's sex: A female has two X chromosomes (XX) and a male has an X and a Y chromosome (XY).

Chromosomal Abnormalities and Miscarriage

Chromosomal abnormalities are the culprits behind most first-trimester miscarriages, but they do not always lead to miscarriage.

In early miscarriages, a baby with chromosomal errors may stop developing because vital genetic information is missing or because the chromosomal arrangement causes the mother's immune system to terminate the pregnancy. Researchers believe that chromosomal abnormalities are behind 60 to 70% of first-time miscarriages.

In these cases, the error is usually a random anomaly and the woman will go on to have a normal subsequent pregnancy.

Other Possible Results of Chromosomal Abnormality

One possible result of a chromosomal abnormality is Down Syndrome. This condition results from having three copies of chromosome 21. Some of the common traits of Down Syndrome are small stature, an upward slant to the eyes, low muscle tone, and a deep crease across the middle of the palm, according to the National Down Syndrome Society. One out of every 691 babies in the U.S. is born with Down Syndrome. The reason why this condition occurs isn't clear, but scientists have seen a link between older maternal age and an increased chance of having a baby with Down Syndrome.

Another possible result of a chromosomal abnormality is Trisomy 18, also known as Edwards syndrome. This occurs when the fetus has an extra chromosome 18 (three, instead of two). This occurs in about 1 out of every 2,500 pregnancies and in roughly 1 in 6,000 births in the U.S., according to the Trisomy 18 Foundation. This condition can cause medication complications that are potentially life-threatening, and only about half of babies who are carried to term will be born alive.

One more potential chromosomal abnormality is Trisomy 13, also known as Patau syndrome. Most cases develop when a fetus has an extra copy of chromosome 13 (three, instead of two). This condition can cause severe intellectual disability as well as heart defects, underdeveloped eyes, extra fingers or toes, a cleft lip, weak muscle tone, and brain or spinal cord abnormalities. As a result, many infants with Trisomy 13 die in the first days or weeks of life. It occurs in approximately 1 in 16,000 newborns, according to the National Institutes of Health.

Testing for Chromosomal Abnormalities

Toward the end of your first trimester, you can elect to take a screening test that can tell you your odds of having a baby with one of these chromosomal abnormalities.

The test is based on your age, an ultrasound, and a blood test. It's usually performed around week 12 of pregnancy.


"Chromosome Abnormalities." National Human Genome Research Institute (2016).

"What is a Gene?" Genetics Home Reference, National Library of Medicine (2016).

"What Is Down Syndrome?" National Down Syndrome Society.

"What IsTrisomy 18?" Trisomy 18 Foundation.

"Trisomy 13." Genetics Home Reference, National Library of Medicine (2016).

"First Semester Screening for Down Syndrome and Trisomies 13 & 18." Johns Hopkins Medicine (2013).

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