Meiosis
Chromosome abnormalities usually happen as a result of an error in cell division. "Meiosis" is the name used to describe the cell division that the egg and sperm go through when they are developing. Normally, meiosis causes a halving of chromosome material, so that each parent gives 23 chromosomes to a pregnancy:

The result is an egg or sperm with only 23 chromosomes. When fertilization occurs, the normal 46 total number of chromosomes results. If meiosis does not occur properly, an egg or sperm could end up with too many chromosomes, or not enough chromosomes. Upon fertilization, the baby could then receive an extra chromosome (called a trisomy), or have a missing chromosome (called a monosomy):

While pregnancies with a trisomy or a monosomy may go to full-term and result in the birth of a child with health problems, it is also possible that the pregnancy may miscarry, or that the baby is stillborn (not born alive), because of the chromosome abnormality. In studies of first trimester miscarriages, about 60 percent (or more) are chromosomally abnormal. In studies of babies who are stillborn, 5 to 10 percent have a chromosome abnormality.

Mitosis
"Mitosis" is the name used to describe the cell division that all other cells, besides the egg and sperm, go through when they are developing. Normally, mitosis causes a duplication, followed by a halving of chromosome material, so that each cell doubles the number of chromosomes to 92, and then splits in half, so that the normal total of 46 is resumed. Mitosis begins after fertilization:

This process repeats itself, until the entire baby is formed. Mitosis continues throughout our lifetime, to regenerate new skin cells, new blood cells, and other types of cells that are damaged or that simply die off.

During pregnancy, an error in mitosis can occur, just like the error previously described in meiosis. If the chromosomes do not split into equal halves, the new cells can have an extra chromosome (47 total) or have a missing chromosome (45 total). This is another way a baby can be born with a chromosome abnormality. Errors in mitosis are responsible for some cases of mosaicism.

Maternal age
When a mother will be 35 years of age at delivery (or older), she may be referred for genetic counseling or for prenatal diagnosis such as an amniocentesis because of her age. There is a difference in the way that eggs and sperm are made. Women are born with all the eggs they will ever have, and over time, there are fewer and fewer available in the ovaries. If a woman is 35 years old, the eggs in the ovaries are also 35 years old. The risk for a baby to be born with a chromosome abnormality increases with the age of the mother. Some scientists and physicians believe that this is because the eggs are aging, and may have the incorrect number of chromosomes at the time of fertilization. Errors in meiosis may be more prone to happen as a result of the aging process.

Men, on the other hand, produce new sperm continually. Therefore, if a man is 35 years of age, his sperm are not 35 years of age. There is, therefore, no increased risk for chromosome abnormalities to occur based on the age of the father. This does not mean that chromosome errors do not happen in the sperm cell. They do. They are just not associated with the age of the father.

Environment
Many parents who have a child with a chromosome abnormality search through their entire life and wonder whether the various environmental exposures they have had over the years contributed to having a baby with a chromosome abnormality. To date, there is not anything specific in the environment, such as x-rays, medications, foods, microwave ovens, etc., that has been found to be the reason for a baby to be born with a chromosome abnormality. In fact, most parents who have a child with a chromosome abnormality, when compared to those parents who do not have a child with a chromosome abnormality, have no difference in habits, lifestyles, or environmental exposures.

There is some evidence accumulating that how a woman's body processes the B vitamin folic acid, may have something to do with why chromosome abnormalities happen. Those women who do not process this vitamin completely, may have a predisposition to having a child with a chromosome abnormality. This has not been proven, but knowing that this is a possibility, gives women of reproductive age a good reason to take a multivitamin with folic acid (before becoming pregnant) and prenatal vitamins during a pregnancy to reduce this potential risk.