That depends on the baby... my oldest daughter self weaned when I was like 9 weeks pregnant, i was told that the milk changes its flavor and they usually don't like it.
I'm pregnant again, let's see if my baby weans or not. She's still nursing but I'm only 4 weeks along :)
One of my kids self weened unrelated to pregnancy. I had supplemented with a bottle and that was easier I guess. He just stopped taking the breast. It was a quick and easy transition. My second probably would never have stopped had I not cut him off! LOL Try offering other forms of drink out of sippy cups and just nursing at certain times like before bed and in the morning. Then give up in the morning. And your toddler will probably start to move on. I know that it becomes more about mommy comfort than nutrition and if they are getting lots of mommy cuddles -- they are fine. :>) good luck and congrats on baby number 2
My daughter was 8 months when I got pregnant with my son. She nursed until 11.5 months and then we switched to milk because I had no supply left and she was super hungry. Some women nurse throughout pregnancy and some kids are fine to dry nurse for a while, mine just wasn't.