No need to wean while pregnant, if you keep up your nutrition. I had 7 kids, approx. 2 yrs apart and nursed each for three yrs. They are all healthy adults now! Your body will produce colostrum for the new baby just fine.
Your milk supply will decrease some, and change flavor, so some toddlers will prefer the new interesting tastes and textures of other foods, while others will prefer to stick with nursing as their main sorce of nutrition. Either choice is fine.
There will be a period of adjustment when your breasts get sore when you nurse, just as when you 1st started nursing your first, but this is temporary, and it is nice to get the discomfort over with before you have a hungry new baby to deal with!
Added bonuses:
You will never leak with two nurslings.
Labor will be shorter, more efficient, less likely to go overdue. It will not cause premature labor however, unless you are not eating well. Most prematurity is caused by impatient Drs or malnourished mothers. (there is a big difference between MALnourished & UNDERnourished. You can eat tons of "food" and still be malnourished, it is quality, not quantity that is important here. See blueribbonbaby.org.)
You won't have to worry about what mischief the toddler is getting into while you are busy with the baby, they will be both happily nursing!
They will bond better and not be jealous. It is so sweet to see your precious babies contentedly nursing with their arms around each other!
Happy nursing!
M., CPM