Usually, there is nothing a woman or her doctor can do to prevent it. There are a number of factors that are known to cause first-trimester miscarriages. The most important of these is a chromosomal abnormality in the fetus. Chromosomes are the tiny thread-like structures in each cell that carry our genes, which dictate all traits from eye color to the workings of our internal organs. Each person has 23 pairs of chromosomes, or 46 in all, with one chromosome per pair coming from the mother and one from the father.
Most chromosomal abnormalities result from a faulty egg or sperm cell.
Before pregnancy, egg and sperm cells divide to form cells with 23 chromosomes. Some-times, the cell splits unevenly in egg or sperm cells with one too many or one too few chromosomes. If a pregnancy occurs involving an egg or sperm cell with the wrong number of chromo-somes, the embryo has a chromosomal abnormality. These embryos often do not survive, leading to miscarriage. Chromosomal abnormalities become more common as a woman ages, which is why women over age 35 are at higher risk of miscarriage than younger women.