The causes of miscarriage are not well understood. Most of the miscarriages that occur in the first trimester are caused by chromosomal abnormalities in the baby. Chromosomes are tiny structures inside the cells of the body that carry many genes. Genes determine all of a person's physical attributes, such as sex, hair and eye color, and blood type. Most chromosomal problems occur by chance and are not related to the mother's or father's health.

Miscarriages are also caused by a variety of other factors, including:

• Infection.
• Exposure to environmental and workplace hazards such as high levels of radiation or toxic agents.
• Hormonal problems.
• Uterine abnormalities.
• Incompetent cervix (the cervix begins to widen and open too early, in the middle of pregnancy, without signs of pain or labor).
• Lifestyle factors such as smoking, drinking alcohol, or using illegal drugs.
• Disorders of the immune system, including lupus.
• Severe kidney disease.
• Congenital heart disease.
• Diabetes that is not controlled.
• Thyroid disease.
• Radiation.
• Certain medications, such as the acne drug Accutane.
• Severe malnutrition.

In addition, women may be at increased risk for miscarriage based on their age. Studies show that the risk of miscarriage is 12% to 15% for women in their 20s and rises to about 25% for women at age 40. The increased incidence of chromosomal abnormalities contributes to the age-related risk of miscarriage.

There is no proof that stress or physical or sexual activity causes miscarriage.

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