Uterine (womb) abnormality - eg. a septum down the middle. This condition is found in 2-4% of the normal population. Whether this type of problem is actually to blame for recurrent miscarriages hasn’t been proven, and the risks of surgery to correct the problem must be weighed against any potential benefit.
Fibroids - whorls of normal uterus tissue growing in the muscle, sometimes causing miscarriage. Occasionally an operation to remove fibroids can be performed.
Cervical incompetence (weakness) - may cause miscarriage in 2nd trimester (after 13 weeks).