I agree with Heather. I didn't have any until 8wks on the 1st pg, then I did every visit. After I lost him at 12wks(found out it was a he during genetic tests) and became pg again, they wanted one done asap. 13wks is devastating. You think it's all ok going into the 2nd trimester. I am so sorry for her/your loss.
I think it varies. I've had two miscarriages, and the first time, I didn't have an transvaginal ultrasound until 8 weeks; but the second time, since I had already had one miscarriage, they started doing weekly u/s starting at 5 1/2 weeks.
I'm so sorry to hear about your daughter-in-law's situation. A transvaginal u/s can irritate the cervix a bit, since the cervix is much more sensitive when you're pregnant, but I don't think it had anything to do with the miscarriage. They say most women have at least one miscarriage in their lifetime; and having one, does not increase the risk of having another one. In the majority of cases, the miscarriage is caused by a genetic abnormality (the embryo will grow until it gets to a stage in the development where that genetic material is needed). However, since her miscarriage happened after the 12th week, it could be a good idea to have a blood panel done, just to ensure that there are no blood clotting factors involved.
I hope your daughter-in-law recovers quickly, and that she'll be blessed with another pregnancy soon!