Chromosomal abnormalities are caused by errors during cell division, DNA damage from environmental exposures, and in some cases, inherited genetic tendencies that make these errors more likely. Roughly 1 in every 150 live births involves a chromosomal abnormality, though the true rate of occurrence is far higher: a large proportion of embryos with chromosomal errors never implant or are lost to early miscarriage.
These abnormalities fall into two broad categories. Numerical abnormalities mean a person has too many or too few chromosomes. Structural abnormalities mean pieces of chromosomes are missing, duplicated, flipped, or swapped between chromosomes. The causes behind each type are distinct.
Errors During Cell Division
The most common cause of chromosomal abnormalities is a process called nondisjunction, where chromosomes fail to separate properly when cells divide. During normal cell division, paired chromosomes are pulled apart to opposite ends of the cell by tiny protein fibers. In nondisjunction, this separation fails, so both copies get dragged to the same side. The result is one cell with an extra chromosome and one cell missing a chromosome.
This can happen at two different stages of egg or sperm production. When it occurs during the first round of division, every resulting sex cell ends up abnormal, either carrying an extra chromosome or missing one. When it occurs during the second round, only half of the resulting cells are affected; the other half are normal. Either way, if an abnormal egg or sperm goes on to participate in fertilization, the embryo will have the wrong number of chromosomes. Trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), and Turner syndrome are all consequences of nondisjunction.
How Quality Control Fails
Cells have a built-in safety system, sometimes called the spindle assembly checkpoint, that is supposed to pause division until every chromosome is properly attached and aligned. Think of it as a last-minute inspection before the chromosomes are pulled apart. When this checkpoint works well, it catches attachment errors and gives the cell time to fix them. When it doesn’t, chromosomes can be distributed unevenly.
Research in egg cells has shown that this checkpoint is less strict than in other cell types. Misaligned chromosomes can exist even while the cell is already progressing toward division. Disabling a key checkpoint component after division is already underway increases the rate of abnormal eggs by up to 30%. This inherent leniency in egg cells helps explain why chromosomal errors in eggs are far more common than in sperm or in ordinary body cells.
What Causes Structural Changes
Structural chromosomal abnormalities, such as translocations, deletions, and inversions, originate from a different mechanism: physical breaks in the DNA strand. Double-strand breaks are among the most dangerous forms of DNA damage, and they happen regularly from normal metabolic processes, radiation exposure, and chemical agents. The cell has repair systems to fix these breaks, but the repair doesn’t always go right.
In a translocation, for example, breaks occur on two different chromosomes at roughly the same time. Instead of each chromosome repairing itself correctly, the broken ends from different chromosomes get joined together. This “illegitimate repair” happens because the broken DNA ends from separate chromosomes are physically close to each other in the nucleus. The result is a rearranged genome. Some translocations are harmless if no genetic information is lost, but others can activate genes that drive cancer or disrupt normal development.
Maternal Age Is the Strongest Risk Factor
The link between a mother’s age and the risk of chromosomal abnormalities in her baby is one of the most well-established findings in genetics. The risk rises exponentially with each year. For trisomy 21 specifically, the rate is roughly 11 per 1,000 pregnancies at age 35, climbs to about 15 per 1,000 at age 40, and reaches 37 per 1,000 at age 45. The overall odds of any chromosomal abnormality increase by roughly 1.16 times for each additional year of maternal age.
The biological explanation centers on the eggs themselves. Women are born with all the eggs they will ever have, and those eggs sit suspended in an incomplete state of cell division for decades. Over time, the protein structures that hold paired chromosomes together (called cohesins) degrade. As these molecular “glue” proteins weaken, chromosomes are more likely to separate prematurely or unevenly when cell division finally resumes at ovulation. This age-related cohesin loss is considered the primary driver behind the steep increase in nondisjunction in older mothers.
Interestingly, not all chromosomal abnormalities track with maternal age. Research has found that trisomy 13, Turner syndrome, triple X syndrome, Klinefelter syndrome, and structural chromosomal rearrangements show no significant correlation with how old the mother is at conception.
Paternal Age and New Mutations
While maternal age gets the most attention, a father’s age matters too, though the mechanism is different. Unlike eggs, sperm are produced continuously throughout a man’s life. The stem cells that generate sperm divide roughly every 16 days, and each division creates an opportunity for copying errors. Research shows that children acquire approximately 1 to 2 additional new mutations for every extra year of the father’s age at conception.
There is also a phenomenon called “selfish selection” that occurs in aging testes. Certain mutations give individual sperm-producing stem cells a growth advantage over their neighbors, causing them to multiply into expanding clusters. This means the mutant sperm cells become disproportionately represented in the ejaculate over time. These specific mutations are linked to a group of conditions known as paternal age effect disorders, though they tend to involve single-gene mutations rather than large-scale chromosomal rearrangements.
Environmental and Chemical Exposures
External agents can damage chromosomes directly by breaking DNA strands or interfering with cell division. Ionizing radiation (from X-rays, nuclear fallout, or certain occupational exposures) is one of the most potent causes of double-strand DNA breaks, which as noted above can lead to structural rearrangements. Hyperthermia, or sustained high body temperature, can also disrupt normal chromosome behavior during cell division.
Chemical exposures with known chromosomal effects include organic mercury compounds, polychlorinated biphenyls (PCBs), herbicides, and industrial solvents. Alcohol and smoking are considered two of the leading preventable causes of birth defects and developmental problems. Certain infections during pregnancy, including rubella, cytomegalovirus, and toxoplasmosis, can also damage developing cells. Maternal health conditions like uncontrolled diabetes further raise the risk.
Folate Deficiency and DNA Stability
Nutrition plays a more direct role in chromosomal integrity than most people realize. Folate (vitamin B9) is essential for building and repairing DNA. It provides a chemical building block needed to make thymine, one of the four “letters” of the DNA code. When folate is scarce, cells accidentally substitute a similar but incorrect molecule called uracil into the DNA strand. The cell then tries to remove and replace the uracil, but this repeated cutting and patching can cause double-strand breaks, leading to chromosomal damage.
Folate also plays a second, less obvious role. It helps maintain the chemical tags (methyl groups) that sit on top of DNA and control which genes are turned on or off. When folate levels drop, these tags are depleted, which can switch on genes that should stay silent, including some associated with cancer development. This dual role in both DNA construction and gene regulation makes adequate folate intake one of the more actionable factors in reducing chromosomal instability.
Genetic Predisposition to Errors
Some families appear to have a higher-than-expected rate of chromosomal abnormalities across generations, suggesting that genetic factors can predispose a person’s cells to make division errors. Multiple case reports document families in which different types of chromosomal abnormalities, not just the same one, appear in siblings or across generations. This pattern, called heterotrisomy, supports the idea that some women carry a baseline risk for nondisjunction that is higher than others of the same age.
Researchers have identified specific inherited gene variants in animal models that increase the rate of nondisjunction. In humans, genes involved in chromosome attachment, the proteins that hold chromosomes together, and the cell’s quality-control checkpoint have all been implicated. Mutations in a gene called PLK4, for instance, can cause abnormal spindle formation in early embryos, leading to multipolar cell divisions and complex patterns of chromosomal gain and loss. Defects in the maternal copies of several spindle-related genes have been shown to increase aneuploidy in early embryonic development.
Errors After Fertilization: Mosaicism
Not all chromosomal abnormalities originate in the egg or sperm. Some arise after fertilization, during the rapid cell divisions of early embryonic development. When this happens, the embryo ends up as a mosaic: some cells carry the abnormality and others don’t. The severity of the condition depends on how early the error occurred and which tissues contain the abnormal cells.
Early embryos are particularly vulnerable to division errors because their cell cycle checkpoints are relaxed compared to adult cells. The divisions happen rapidly, with shortened gap phases that leave less time for DNA repair. Cells with damaged DNA that would normally be flagged and destroyed in adult tissue can slip through and continue dividing in an embryo. Additional causes of mosaicism include centrosome defects (often from abnormal fertilization events like fertilization by two sperm), replication stress from the pace of early cell division, and incomplete “trisomy rescue,” where an embryo that starts with three copies of a chromosome loses one copy in some cells but not others, creating a patchwork.