Reasons for a fetus not to grwo properly?

I'm so sorry for your loss. I miscarried four times. I went for my 12 week check up and the doctor couldn't find a heartbeat three of the times. All three times the fetus stopped growing at 9 weeks. I often wonder what happens in development at 9 weeks that caused the fetuses to die then I guess something doesn't form properly for the fetus to sustain life. It is just kind of nature's way of making sure we don't have babies that are deformed or with brains that didn't grow properly. You will never find a specific reason. Again, I'm sorry for your loss.