I have it. It's called unicornuate uterus, and it's pretty rare. Bicornuate is much more common.
It's not that it's a "miracle" if you manage to conceive, but from what I understand, it can be very hard for SOME women with UU to carry to term. Depending on how severe the UU is. Since the uterus only is a half, however, you only have one working fallopian tube that actually connects, so it does cut fertility in half.
I don't think I had much trouble with mine. I did have a miscarriage at 6 weeks, but that may not have been because of UU. I DID have an ectopic directly attributable to UU because by some weird less than 1% chance, a sperm jumped the uterus and hit the fallopian tube that wasn't connected to my uterus, fertilized the egg, and then couldn't get out. So I had the tube removed.
I was also told that I should never EVER even consider carrying multiples.
My pregnancy was very normal. Birth, however, was pretty rough, and it took 50 hours of hard labor and four hours of pushing before dd was born. She was stuck in my pelvis, possibly because of a small turn she had to make due to my uterus. When she did, she came FLYING out.
She was post dates but VERY small, probably because she didn't have much room to grow.
My placenta also became stuck and was anterior due to the uterus issues. I nearly had a d/c, but it finally delivered- folded neatly in half. Again, because of the UU. Also, because of the weirdness, my ctx never showed up on the monitor. My OB nurse didn't believe I was in labor for a while.
This issue does apparently run in families. They do say that once the first pregnancy and birth is complete, the uterus stretches and makes subsequent births easier.
UU is not a guaranteed c-section (for my doctor, but YMMV), but a LOT of women with UU deliver via section partly to avoid labor issues like mine if their doctor is concerned, and partly because UU leads to difficulty with the fetus turning since fewer drs are doing breech deliveries these days. My dd didn't turn on her own and I ended up getting the Webster Technique (which did lead to her turning).
As for diagnostics, I don't know how she would have been diagnosed. I got YEARS of pelvic exams and no one ever noticed a thing. It was finally diagnosed through HSG because I went to a fertility doctor to conceive and he did a quick "let's just make sure all is where its supposed to be" ultrasound as a formality and well...it wasn't.