There are many different reasons for having a miscarriage but the actual reasons for the miscarriage is often unknown. Approximately 50% of first trimester miscarriages are due to a chromosome abnormality in the fetus, though some people believe the incidence is higher than 50%. The most common chromosomal cause of a miscarriage before 12 weeks is an abnormality such as

• Monosomy X,
• trisomy 16,
• trisomy 21,
• trisomy 13, and
• trisomy 18.

Most chromosomal abnormalities are the cause of a faulty egg or sperm cell, or are due to a problem at the time that the zygote went through the division process.Occasionally a miscarriage can be caused by an inherited variation with the chromosomes. For example, a parent can have a rearrangement (a "translocation") of his or her chromosomes, in which the chromosomes are structured differently.

Causes for miscarriage include:

• Chromosomal anomalies
• Maternal age
• Infections 
• Maternal health problems
• Smoking, 
• Drug use, 
• Malnutrition
• Exposure to radiation or toxic substances
• Maternal trauma
• Cervical incompetence (insufficiency)

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