What can cause a miscarriage?

Came across while surfing the causes of miscarriage:
Approximately 40% of miscarriages have no medical explanation for their occurences. Also 50-60% of first trimester and 20% of second trimester miscarriages are due to chromosomal abnormatlities within the fetus.

What are chromosomes?
Chromosomes are tiny string-like structures in cells of the body that contain the genes. Humans have about 20,000 to 25,000 genes that determine traits like eye and hair color. They also direct the growth and development of every part of the body (3).

Each person normally has 23 pairs of chromosomes, or 46 in all. We inherit one chromosome per pair from our mother and one from our father.

Sometimes, however, a baby can be born with too many or too few chromosomes, or with one or more chromosomes that are missing a piece or are rearranged. These errors in the number or structure of chromosomes can cause a wide variety of birth defects ranging from mild to severe. Some chromosomal abnormalities result in miscarriage or stillbirth.

What causes chromosomal abnormalities?
Chromosomal abnormalities usually result from an error that occurred when an egg or sperm cell was developing. It is not known why these errors occur. As far as we know, nothing that a parent does or doesn't do before or during pregnancy can cause a chromosomal abnormality in his or her child.

Sperm and egg cells are different from other cells in the body. These cells have only 23 unpaired chromosomes. When an egg and sperm cell join together they form a fertilized egg with 46 chromosomes.

But sometimes something goes wrong before fertilization. An egg or sperm cell may divide incorrectly, resulting in an egg or sperm cell with too many or too few chromosomes.

When this cell with the wrong number of chromosomes joins with a normal egg or sperm cell, the resulting embryo has a chromosomal abnormality. A common type of chromosomal abnormality is called a trisomy. This means that an individual has three copies, instead of two, of a specific chromosome.

Down syndrome is an example of a trisomy. Individuals with Down syndrome generally have three copies of chromosome 21. Children with Down syndrome have varying degrees of mental retardation, characteristic facial features and, often, heart defects and other problems.The risk of Down syndrome and other trisomies increases with maternal age.

In most cases, an embryo with the wrong number of chromosomes does not survive. In such cases, the pregnant woman has a miscarriage. This often happens very early in pregnancy, before a woman may realize she's pregnant. Up to 75 percent of first trimester miscarriages are caused by chromosomal abnormalities in the embryo.