One medical definition of a miscarriage is the spontaneous loss of a pregnancy before the 24th week, and I read that it happens as often as in about 25 percent of all pregnancies.  Often, miscarriages take place within the first tri-mester of the pregnancy.  Many miscarriages happen randomly, and the cause is often a problem with how the sperm and egg had combined in the fertilization process.  This does not mean the problem will occur again, and it doesn’t mean that the woman can not get pregnant again.  There are other factors that can cause a miscarriage.  If a pregnant woman has a severe cold, respiratory infection, or diminished immune system, it can contribute to this problem.  Also, drinking alcohol, smoking, or enduring a great deal of stress may lead to a miscarriage.  In some cases, the mother may have an undetected chromosome disorder, but this does not happen frequently.  Many miscarriages occur without any symptoms or signs of any problems.  Frankly, doctors often cannot explain the reasons for this occurrence, but it is rarely the fault of the mother or father, it just happens.