I had multiple MC over several years before DC4 in my 40s. Was getting preg every 2nd or 3rd cycle.
On the advice of consultant, I took very high folic acid (5mg, you can take about 6 of the normal ones if you can't get hold of 5mg ones, I couldn't sometimes even with a prescription due to a supply issue), I was to take them while TTC and till about 20weeks preg. Also high strength fish oil and vitamin d. I was also to take mini aspirin as soon as I got a positive test, and until (I think) about 30 weeks (maybe 34, can't quite remember).
My very next pregnancy was successful. As well as helping in other ways, the thing all the above have in common, is they all have an anti inflammatory action - they reduce inflammation and so reduce the chances of the immune system attacking the embryo, it's placenta and the blood supply to it. Vit d.
A lot of immune and autoimmune factors increase as we get older, some can be tested for, some not. I very much wish that I'd been gluten and grain free while TTC, I do think I might have made a difference, based on the huge health improvements I've had in the 3 years since adopting a paleo/primal diet (grain and gluten free, low sugar).
Therefore, I'd suggest gluten free (can't do any harm, may help), as it does seem to have an inflammatory effect on many many people.