I had my son vaginally, but he was only 7lbs 4oz (he was three weeks early). I had a small first degree tear, and it was no big deal, really. I didn't have U/C at the time (well, hadn't been diagnosed, but now I suspect I might have been very midly flaring during pregnancy), but I did start to flare about
4 weeks after. The tear was healed by that time. What really was a problem were the hemmies I got from pushing. They got really, really bad during the flare.
There's a risk of tearing with any vaginal birth, but the risk is higher if the baby is bigger. FWIW, ultrasounds can be really off when it comes to weight. I've had some friends have babies be off by as much as a pound or pound and a half, but only SMALLER (so the u/s said 9lbs 8oz and the baby was only really 8lbs). I also had a friend vaginally birth an 11lb baby without tearing. The best way to avoid tearing is to ask your OB to massage your perineum as you're pushing. Mine also poured warm water over the area to help it stretch. Even though he wasn't a fat baby, he had a big head (it's always been off the charts).
Personally, I'd try for the vaginal birth. A C-section is major surgery, and is a lot harder to heal from than a vaginal birth, even if you tear.
Be VERY SURE to keep up with your meds after you deliver. It seems the majority of women with UC flare after childbirth, so now is not the time to taper your meds or quit taking them. I'd also use rectal meds as much as you can. I know it'll be hard with a newborn, since they don't sleep for very long stretches, and you have to be up with them.
Status: Remission since May 2009!
Symptoms began in November 2008, ~4 weeks after giving birth to my son
Diagnosed with pancolitis on 1/30/09
Meds: Apriso (4 0.375g pills ONCE!! daily), mesalamine enema twice weekly, Natural Factors Ultimate probiotic 12/12 Formula, multivitamin. Used prednisone (starting dose 40 mg) to get into remission.