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, or possibly miscarriage.

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.

What Are Chromosomal Abnormalities?

Chromosomal abnormalities are differences in the chromosomes that can happen during development. They could be "de novo", or new to the fetus, or inherited from a parent.

Abnormalities are split into two categories: numerical and structural.

Numerical Abnormalities

Numerical refers to the fact that there are a different number of chromosomes that expected; there could be more or less. This is also called aneuploidy. Each scenario has a specific term.

  • Monosomy: A chromosome is missing from a pair.
  • Trisomy: There are three chromosomes instead of two.

Structural Abnormalities

When the makeup of the chromosome presents an issue, this is known as a structural abnormality. There are several ways that structural abnormalities may occur.

  • Translocation: In this instance, one piece of a chromosome is transferred to another. This could be a Robertsonian translocation (one chromosome attaches itself to another) or a reciprocal translocation (two chromosomes are traded).
  • Deletion: There is a deleted or missing part of a chromosome.
  • Duplication: This means part of a chromosome is copied and so there is extra genetic material.
  • Ring: A ring/circle forms as a result of a portion of a chromosome tearing off.
  • Inversion: There is an inversion when a piece of chromosome breaks off and turns upside down, then reattaches itself to the original structure.

Why Do Chromosomal Abnormalities Occur?

Chromosomal abnormalities occur because of cell division that does not go as planned. Typical cell division happens by either mitosis or meiosis.

When a cell, comprising 46 chromosomes, splits into two cells, this is called mitosis. The new cells should also have 46 chromosomes a piece. Bodies are made up of cells that have formed from mitosis.

In meiosis, a cell also divides into two halves, but these halves each have 23 chromosomes. Meiosis produces sperm and eggs in reproductive organs.

If either mitosis or meiosis results in a number of chromosomes than expected, this is considered a chromosomal disorder.

Risk Factors

There are some factors that could potentially increase the chances of a chromosomal disorder. One such consideration is the age of the parents.

As women get older and pass age 35, there is a higher risk of a chromosomal abnormality occurring. Although there is conflicting research and more to learn about advanced paternal age (sometimes defined as ages 40 and older), this, too, has evidence of possible increased risk for chromosomal disorders.

Additionally, environmental factors, like exposure of a fetus to certain drugs, could be involved in chromosomal abnormalities.

Chromosomal Disorders

There are different types of disorders that could arise from abnormal chromosomes.

Down Syndrome

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

The reason for Down syndrome 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 1 in 1,500 at age 20 to 1 in 50 by age 43.

One out of every 691 babies in the United States is born with Down syndrome.

Turner Syndrome

Turner syndrome affects females. It could make them infertile or shorter than average. Other features of the disorder may be swelling of feet/hands, extra neck skin, kidney and heart issues, as well as skeletal abnormalities. Turner syndrome involves the X chromosome and could result in miscarriage or stillbirth.

Klinefelter Syndrome

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.

Trisomy 13

Trisomy 13 or 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.

Trisomy 18

Trisomy 18, sometimes called Edwards syndrome, occurs in one of every 2,500 pregnancies and about one of every 6,000 births in the United States. 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.

Triple X Syndrome

Also known as Trisomy X or XXX syndrome, Triple X syndrome affects females. It could make them taller than average with weaker muscles, have issues with speech, or be uncoordinated. Many girls with with this disorder end up with productive lives.

Triple X syndrome affects 1 in 1,000 females.

XYY Syndrome

XYY syndrome affects 1 in 1,000 males and is caused by the presence of an extra Y chromosome in their cells. Males with the disorder could be taller than average, have speech processing disorders, or be uncoordinated. It may also cause behavioral issues, hand tremors, and weaker muscles. XYY syndrome symptoms vary, but mostly those who have it experience productive lives.

Fragile X Syndrome

Fragile X syndrome or Martin-Bell syndrome results from an X chromosome gene change. It can cause intellectual and learning disabilities, social/behavioral issues, and developmental delays.

Cri-Du-Chat Syndrome

When a piece of chromosome 5 is absent, it is categorized as Cri-du-chat (cat's cry) syndrome or 5p- (5p minus) syndrome. The abnormality is named as such because infants who have it may give a cry that mimics a cat's. Symptoms of Cri-du-chat may include small head size, weak muscle tone (for infants), delayed development, low birth weight, heart defect, or intellectual disability.


Chromosomal abnormalities can lead to complications during pregnancy. Two such complications are miscarriage and molar pregnancy.


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 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.

Molar Pregnancy

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 it 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.

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.

A Word From Verywell

Learning about chromosomal abnormalities can be overwhelming, especially if they of primary concern during pregnancy. While there are tests that can be performed during the first trimester to gain insight into development or complications, genetic counseling prior to conception could also offer more information that is valuable for those trying to have a baby.

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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.
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