You should not have to endure a 3rd miscarriage, before the doctor's do some specific testing
to see if your hormone levels during pregnancy
are too low to allow the pregnancy to continue. Since you talked about having a series of blood work done and all came back negative, I am wondering if they checked your levels for estrogen, progesterone, etc.
Again, I am so sorry that you have been through this. This is heartbreaking. Best of luck to you.