Miscarriages are sometimes determined by a chromosomal imbalance. Some times there is nothing that can be done to sustain a pregnancy if it is not meant to be. However other then that if there are drugs involved that can cause someone to have a miscarriage,some people who may not be in optimum health can have a higher risk of miscarriage, also if a person has had abortions or problems with pre term labor they are at high risk of miscarriage. In general you want to avoid stress,drugs,alcohol and straining yourself like working out excessively during pregnancy. If you did not work out prior to being pregnant you do not want to start mid pregnancy it is better to play it safe and eat healthy and attend all your prenatal visits as well as take vitamins and this will be the best you can do.