Chromosomal Abnormalities in Pregnancy

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

A chromosomal abnormality occurs when a fetus has either the incorrect number of chromosomes, incorrect amount of DNA within a chromosome, or chromosomes that are structurally flawed. These abnormalities may translate to the development of birth defects, disorders like Down syndrome, miscarriage, or may be a normal variant.

Understanding Genes and Chromosomes

Your body is made up of cells. In the middle of each cell is a nucleus, and inside of the nucleus are chromosomes. Chromosomes are important because they contain genes which determine your physical characteristics, your blood type, and even how susceptible you will be to certain illnesses.

Each cell in the body typically contains 23 pairs of chromosomes — 46 in all — each of which contains roughly 20,000 to 25,000 genes. Half of your chromosomes come from the egg of your mother and half come from the sperm of your father.

Of the 23 pairs of chromosomes, the first 22 pairs are called autosomes. The final two chromosomes, meanwhile, are called allosomes. Also known as sex chromosomes, allosomes determine an individual's sex and sexual characteristics. A female has two X chromosomes (XX) while a male has an X and a Y chromosome (XY).

Chromosomal Abnormalities and Miscarriage

Chromosomal abnormalities are one of the primary causes of miscarriage during the first trimester. In early miscarriage, chromosomal errors can prevent a fetus or embryo from developing normally. When this happens, the immune system will often respond by spontaneously terminating the pregnancy, but some miscarriages require assistance (through medication or a procedure) for the tissue to pass from the uterus.

Research suggests that chromosomal abnormalities are behind 60% to 70% of first-time miscarriages. In most cases, the error is a random anomaly, and the woman will go on to have a normal subsequent pregnancy.

A significant number of miscarriages are caused by a type of disorder where there are three copies of a chromosome instead of two. This is called a trisomy. Examples are trisomy 16 and trisomy 9, which together account for around 13% of all first-trimester miscarriages. This type of chromosomal abnormality occurs more often with increasing age of the woman. 

In other cases, a chromosomal abnormality may lead to a rare condition called a molar pregnancy. During a molar pregnancy, tissues that were meant to form into a fetus instead become an abnormal growth on the uterus. There are two types of molar pregnancy:

  • A complete molar pregnancy is caused when the egg has no genetic information, and is fertilized by one or two sperms. Due to the lack of genetic information from the mother's side, the fertilized egg develops a placenta that looks like a cluster of grapes without an accompanying fetus.
  • A partial molar pregnancy happens when an egg with genetic material is fertilized by two sperms. It causes the development of an embryo that has multiple copies of chromosomes, forms some abnormal placental tissue and usually does not survive.

Down Syndrome and Other Chromosomal Abnormalities

One of the most well-known chromosomal disorders is Down syndrome caused by an extra copy of a chromosome called chromosome 21. It is for this reason that we also refer to the disorder as trisomy 21.

Some of the common traits of Down Syndrome are developmental delay, small stature, an upward slant to the eyes, low muscle tone, and a deep crease across the middle of the palm.  One out of every 691 babies in the U.S. is born with Down syndrome.

The reason for the disorder is not entirely clear, but scientists have noted, among other things, a common link between older maternal age and development of the syndrome. Risk has been shown to increase exponentially as a woman ages, from one in 1,500 at age 20 to one in 50 by age 43.

In addition to trisomy 21, there are other disorders related to the presence of an extra chromosome, chief among them

  • Trisomy 18, also known as Edwards syndrome, occurs in one of every 2,500 pregnancies and about one of every 6,000 births in the U.S. This disorder is characterized by low birth weight, a small abnormally shaped head, and other life-threatening organ defects. Edwards syndrome has no treatment and is usually fatal before birth or within the first year of life.
  • Trisomy 13, also known as Patau syndrome, can cause severe intellectual disability as well as heart defects, underdeveloped eyes, extra fingers or toes, a cleft lip, and brain or spinal cord abnormalities. Patau syndrome occurs in one of every 16,000 births, with infants usually dying within the first days or weeks of life.
  • Klinefelter syndrome, also known as XXY syndrome, is the result of an extra X chromosome in males. It is associated with high rates of sterility and sexual dysfunction. It typically goes unnoticed until puberty when it is characterized by weak musculature, tall stature, little body hair, and small genitalia.

By contrast, the addition of an extra Y in males (XYY) or an extra X in females (XXX) does not result in any specific physical characteristics or health concerns. While some of these children may have learning difficulties, they typically develop normally and are able to conceive children.

Chromosomal Testing

Toward the end of your first trimester, you can elect to have a screening test which can provide you the probability of your baby having a chromosomal abnormality. The testing can be divided into screening and diagnostic tests. Screening tests include ultrasound and blood tests, such as a panel of biomarkers or testing of circulating placental DNA. Diagnostic tests include chorionic villus sampling or amniocentesis, which are both minimally invasive exams.

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Article Sources

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Additional Reading

  • National Human Genome Research Institute: National Institutes of Health. "Chromosomal Abnormalities." Bethesda, Maryland; updated January 6, 2016.