I didn't know that I had HMS when I was having my daughter, and had HMS related symptoms.....
Waters broke a couple of weeks early, labour only lasted 4 hrs, felt every stitch of my pereneal repair as the anaesthetic didn't work.
To be honest, it made for quite a hard time, as I was 1cm dilated when I was examined, and then everything kicked in and the midwife refused to believe I was in labour, she was quite horrible and very belittling and told me that I didn't know what pain was - it was horrible really, it's only when another midwife took over 3 hrs later and I kicked up such a huge fuss, that she examined me and told me I was fully dilated and could push whenever I was ready.
Then when I was stitched, another midwife came to oversee my repair and because I kept wincing and ouching, despite them keep giving me more and more anaesthetic, she eventually started to lose her patience with me, and went "here, you've no doubt been using this already" and shoved the gas an air in my face, and the other midwife stepped in, and said "she's just given birth without an ounce of pain relief, she doesn't need that now, does she?" and took it back again.
All in all though - I loved giving birth, it was fab....
...the fact that you are aware of your condition and the possible implications that it could have on your birth experience, should stand you in good stead.