If you're interested in reading up about this, there's a book called adventures in tandem nursing published by la leche league. You can get it on their website or apparently if you have a local branch they might have a copy they can lend you.
I found it useful and it's highly recommended by many on here.
My DD weaned herself when I was about 22 weeks pregnant (I'm 27 now). When I got pregnant I was feeding her three times a day. She's 20 months now so I think she was about 15-16 months when I found out I was pregnant.
I deliberately dropped her afternoon feed in the first couple of months, to be honest I can't remember why. Then she stopped looking for her morning feed, so I gave her a cup of cows milk. Then about a month ago she lost interest in her bedtime feed so I stopped offering, always intending that if she made any moves to asking for a feed I would give her. But she wasn't fussed.
I think most people see a drop in their supply. In my case, I think DD had been dry nursing for 1-2 months before she stopped feeding. I have colostrum now, but I think it came in too late to 'save' breast feeding for us.
I am sad that DD has weaned, but it was a really nice gradual process. I've never had to refuse her a feed and, although things were sped up by my drop in supply, I feel I've done things to her timetable.
Oh, I almost forgot, feeding did start to hurt. Never toe curlingly unbearable but just very uncomfortable, I think more so once she was dry nursing. Again, I think most women experience discomfort of some sort, with some women finding it very very uncomfortable.
I hope your pregnancy goes well and you and your DD come to a mutually acceptable solution re continuing with nursing. 