Yes, it is hereditary but does not mean the child will get it and I don't know a stat to give you with the chances.  Moya-Moya can come at any stage in life, 9 months, 16 years old, 40 years old.  You just never know when the diagnosis will be.  Moya-Moya effects everyone differently.  My daughter was 2 years old when she was diagnosed. Since her stroke which was three years ago. She had not had any problems.  My daughter also has Down Syndrome so the chances of Moya-Moya are higher.  My advice to you would be try not to worry about it, I know it is easier said than done. I still worry about my daughter.  If the MRI comes back with no signs that is great but as you know things can always change.  If the doctors do see something, I would seek a second opinion and find a doctor who knows about the disease.  We are fortunate to have a great doctor who knows about the disease and has been very careful with our daughter.   When friends ask me about Moya-Moya I always tell them that my daughter's chances of having Moya-Moya are a little bit higher because she has Down Syndrome.  On the other hand there are a lot of people who have the disease who are healthy individuals.  You yourself who is healthy never has a problem could wake up tomorrow, not feel right, go to the hospital and you are diagnosed with Moya-Moya.  You just never know. Congratulations on the adoption and please keep us updated with the results of the MRI or if you have any more questions.

It is my understanding that things could look fine right now but still develop later in life.  My son was 16 when he was diagnosed and had surgery.  I can look back at least several years and find symptoms that we and the doctor attributed to other things.  If I were in you position, adopting a child from a mother with moyamoya, I would familiarize myself with the myriad symptoms of moyamoya and then keep an eye out.  You will probably also want to educate the pediatrician on the disease so he will be willing to do testing if you later feel it is necessary.

Enjoy your new daughter!