Did she have her progesterone level checked?
I also found this at a website describing reasons for miscarriages:
Physical Problem with the Uterus or Cervix
Some women have a uterus that does not have the usual shape. Others have a cervix that may be weakened by a number of causes, including multiple D&C procedures or their mother taking DES when she was pregnant (although note that DES was discontinued in 1971 and most DES daughters are leaving their childbirth years behind.) Both of these problems can cause early labor, usually during a critical period from 12–24 weeks. This cause is responsible for 12 percent of miscarriages during this time period. As the baby grows, especially during the very rapid growth spurt during this time frame, the irregularly shaped uterus may not be able to expand or the weak cervix may start to open up and let the baby out. There are treatments for both of these that are quite effective—corrective surgery on the uterus and a cervical stitch that holds the cervix closed. This problem WILL REOCCUR if not treated.
http://www.pregnancyloss.info/causes.htm#uterus