Miscarriage

Hi ... I'm a happily married 26 year old! :) I've been pregnant 4 times, 3 were miscarriages and 1 successful pregnancy with my son. My husband and I really want another baby but I'm having a difficulty carrying a pregnancy past 6 weeks. My doctor told me I probably have a thin uterus but doesn't want to do any testing unless I have a 4th miscarriage, because she says I'm young and healthy and not every flower blooms so just keep trying.... Any advise on how to make the body extra ready to carry a successful pregnancy ? I'm desperate and a little discouraged.