E., I am so sorry for your losses. It sounds like maybe your hormone levels might be little low -- enough so you can get pregnant, but not enough to sustain the pregnancy until the placenta takes over?
I breastfeed long-term, and I've had miscarriages when my cycle begins to come back and my hormone levels increase enough so that I can get pregnant, but have not yet increased enough to sustain a pregnancy until the placenta can take over. The first time it happened, we didn't know why, but did some research and began to suspect. After a few more months I got pregnant again and everything was fine. After the second child, we waited longer before allowing me to get pregnant (we went on a trip and the medication and pregnancy wouldn't be wise to mix), and everything went fine. Next time, I miscarried. Then, my midwife had the idea it was my hormone levels still low and suggested I start taking Mexican Wild Yam. I did, and everything went fine. The next time after that, I took the Wild Yam again when I was pregnant and felt a miscarriage was about to happen, and everything turned out fine.
So, progesterone may help, and also herbs. Wild Yam is a good one for it to increase fertility (after ovulation - not before) and to increase your hormones, also Red Clover.
The doctors will usually understand the drugs and not the herbs, the midwives will usually understand both pretty well, but especially the herbs. If you get a doctor that understands both as well as the midwives, you have a treasure as great as the midwives!
You might try calling a few midwives to get their opinion on it. I don't know about the CM (Certified Midwife), but the home-birth midwives, CPMs (Certified Professional Midwife) will (I've only known a few, but they are really knowledgeable and great about all the childbearing and related issues.
Good luck and keep smiling :)