I had a septum in my uterus, and carried my son to full term. Later after a miscarriage, I had a D&C and they removed the septum. My son just pushed the septum aside as he grew. But the reasoning for removing the septum is that it has poor blood supply, and if an embryo implants on it, the embryo might not get enough blood and the pregnancy fail. I don't think there are reliable numbers where someone did a test comparing a lot of women known not to have septums with a lot of women known to have them, and followed both groups, and then determined the pregnancy success rates of each group. That this kind of test hasn't been done is probably because the only reason a woman even finds out she has a septum is if some diagnostics are being done because she has a problem of one kind or another in the uterus, and so it wouldn't be a clean, random sample if they watched that group of women. (Those would be women with septums, yes, but they would be women who had already had a known uterine issue. Which doesn't compare fairly to a randomized group.)
Anyway, the long and short of it is that nobody knows for sure if the risks are high of miscarriage if you have a septum. (Some women probably go their whole lives with normal pregnancies and never even learn they have them.) I didn't have an issue.
ps -- The miscarriage I mentioned was due to twin-to-twin transfusion syndrome, and didn't have anything to do with my septum.