Causes

Problems with the genes or chromosomes

Most miscarriages occur because the fetus isn't developing as expected. About 50 percent of miscarriages are associated with extra or missing chromosomes. Most often, chromosome problems result from errors that occur by chance as the embryo divides and grows — not problems inherited from the parents.

Chromosome problems might lead to:

• Blighted ovum. Blighted ovum occurs when no embryo forms.
• Intrauterine fetal demise. In this situation, an embryo forms but stops developing and dies before any symptoms of pregnancy loss occur.

• Molar pregnancy and partial molar pregnancy. With a molar pregnancy, both sets of chromosomes come from the father. A molar pregnancy is associated with abnormal growth of the placenta; there is usually no fetal development.

  A partial molar pregnancy occurs when the mother's chromosomes remain, but the father provides two sets of chromosomes. A partial molar pregnancy is usually associated with abnormalities of the placenta, and an abnormal fetus.

  Molar and partial molar pregnancies are not viable pregnancies. Molar and partial molar pregnancies can sometimes be associated with cancerous changes of the placenta.

Maternal health conditions

In a few cases, a mother's health condition might lead to miscarriage. Examples include:

• Uncontrolled diabetes
• Infections
• Hormonal problems
• Uterus or cervix problems
• Thyroid disease

What does NOT cause miscarriage

Routine activities such as these don't provoke a miscarriage:

• Exercise, including high-intensity activities such as jogging and cycling.
• Sexual intercourse.
• Working, provided you're not exposed to harmful chemicals or radiation. Talk with your doctor if you are concerned about work-related risks.