Miscarriage can occur in any of the three trimesters, but is actually most common during the first trimester. Generally, miscarriage is defined as fetal loss before 20 weeks of gestational age. Often, early miscarriages (e.g. before 6 weeks) are not recognized as miscarriages, because the woman may not realize that she had been pregnant. Researchers estimate that 50% of all pregnancies are lost to miscarriage in the first 6-8 weeks. Of those, the vast majority are due to genetic defects. Trisomy--or having three copies of a chromosome, when only two copies should normally be present--is one of the most common causes of miscarriage. This is a problem that happens at the time the sperm and egg unite, and is not necessarily relate to a specific genetic problem with the mother or father.
Having one miscarriage does not increase your risk of future miscarriage, but if you have two miscarriages then your future pregnancies are at a slightly higher risk. Other factors that increase the risk of miscarriage include the woman's age (each year over 35 years has more risk), illicit drug use, smoking, diabetes, thyroid disease, and cervix abnormalities. Your primary care doctor
or obstetrician can discuss these risk factors with you. If you have had multiple miscarriages, your doctor can refer you to specialists who can look into other causes.