Third miscarriage

Today I’m about to go through my third miscarriage in less than a year. My husband and I have been trying to conceive for 5 years now and didn’t get our first positive pregnancy test until November 2017. I had a miscarriage in November 2017, April 2018 and now July 2018. I’m devastated and so beyond depressed that We will never have a baby. My doctor has ran all kinds of test on me but doesn’t know why I’m having multiple miscarriages. She thought it was due to low progesterone since I had that with my second one but she had me on vaginal suppositories this cycle and my levels were fine but I miscarried still. We did IUI this cycle that I got pregnant on. We have spent so much money these past two months doing IUI when we really don’t have the funds for it and now just to have both fail. I don’t know what we can do going forward to hopefully prevent this from happening again. I want to know what is causing this. 😔