This is my third pregnancy with nurslings. My experience with each pregnancy so far has been that my supply dips sharply around 7-8 weeks and then basically dries up entirely around 20 weeks.
Last time, I had a 30 month old and a 16 month old when I got pregnant and they both continued to nurse for the duration of the pregnancy. Thankfully, the older guy weaned maybe 6-8 weeks after the baby was born because his nursing was driving me insane. I'm think it was a combination of the shape of his mouth and the hormones because this time I have a 38 month old and a 14 month old and again it's just the older guy who's driving me crazy. I am hopeful that he will wean soon because I don't know if I can stick out a whole pregnancy like this, even though he's only nursing morning and night. It was actually starting to get on my nerves before the pregnancy but now it grates terribly! I've been talking to him for several months about how nursing is for very little children, and as children get older their mouths change and they are not shaped for nursing anymore, and reminding him about how his big brother stopped nursing when he was around this age. I have to say he's gotten much less aggressive about insisting on nursing and much more likely to pull off because he sees I'm uncomfortable or to agree ahead of time to "two minutes" and then actually pop off after not much longer than that so I have hope.
I'm not sure what I'm going to do with #3. When #2 was born, #1 was just 14 months old -- there was no way in good conscience I could wean him during that pregnancy and there were no jealousy issues because they were both babies together. When #3 was born, #2 had major issues wanting to nurse all day, which I just couldn't do -- (1) my milk supply takes a while to establish and I couldn't let him take all the milk while baby slept; (2) it was horrible for him when spent all day trying to nurse and not doing anything else; (3) I really didn't want to jeopardize #1's process toward weaning, and I know he would have insisted on nursing as often as #2 if it worked for #2; and more reasons. But the tension created a lot of stress the first couple of months. I'm tempted to avoid that by weaning #3 before I give birth, but he's still so little! So I'm conflicted.