There are a lot of causes for missed abortins including genetic and chromosomal disorders, infection, Hormonal
disturbance including thyroid
and lot more. Sometimes the cause cannot be determined.
However, in order to prevent the further mishap, you have to be thoroughly investigated for the above.
To start of, you can get blood group
, TSH, S. Prolactin, Blood sugar, Urine r/m
If no abnormality detected go for a set of higher investigations after consulting your doctor
Chromosomal cause cannot be found at p[resent as it is carried on the tissue obtained at the time of D+C
All the very best.