Probability of Miscarriage

Miscarriage (spontaneous abortion) occurs in approximately 10-20% of known pregnancies, with the true rate likely higher since many miscarriages occur very early, before a woman even knows she's pregnant. When including these very early losses, the total rate may be closer to 30-40%. About 80% of miscarriages occur in the first 12 weeks (first trimester).

The most common cause is chromosomal abnormalities in the embryo, accounting for about 50-60% of first-trimester miscarriages. Risk increases significantly with maternal age: about 10% at age 20, 15% at age 35, 25% at age 40, and over 50% at age 45. Other risk factors include prior miscarriage, chronic conditions (diabetes, thyroid disorders, PCOS), uterine abnormalities, infections, smoking, heavy alcohol use, and obesity.

Most miscarriages cannot be prevented, and having one miscarriage does not significantly increase the risk of another (the recurrence risk after one miscarriage is about 20%, similar to the baseline rate). However, recurrent miscarriage (three or more consecutive losses) affects about 1% of couples and warrants investigation for underlying causes such as antiphospholipid syndrome, uterine abnormalities, or chromosomal translocations in either parent.