I haven't done it (just have the one DS), but I've read that it's fine. Moms can nurse through the entire pregnancy, but are often sore and milk may dwindle or stop. When the new baby is born, mommy's body makes the milk for the youngest (you know how milk is age-appropriate). There were some articles in New Beginnings the issue before this latest one. (I haven't read the latest one yet to see if there's more there.) I know that breast stimulation can induce contractions, so if mom's worried about pre-term labor it might not be a good idea. I've even seen a maternity-nursing top (the caption read something to the effect, "so you got knocked up before you weaned the first, huh? =) ") LLL has a book called "Adventures in Tandem Nursing" that would probably have information for you.
I've always thought it would be a good idea to tandem nurse. I think it would help the older child adjust to the younger one. "No, you can't nurse any more, but the baby can 24/7." Obviously, that's not what you say to the older one, and I haven't been that road so this is not the voice of experience. My ideas may very well change when I get to that point in my life. That's what I've read and heard. Good luck!
My DD is 16 months and I am currently 38.5 weeks. I got pregnant much sooner than expected and worried about our nursing relationship but we haven't had any problems per se. I know my supply is low but she doesn't seem to mind. She nurses 3-5 times a day. I do expect that once the new baby comes with the new milk she will want to nurse more often
FYI - I have had some pre-term labor and false labor but never while nursing, and all was solved at home. I have stronger contractions from orgasm than while breastfeeding so don't let that worry you.
I nursed through my last pregnancy dd was 13 months when I got pregnant and then tandem nursed. We didn't have any preterm labor but it did get quite painful and I needed ALOT of distractions during nursing.
I'm nursing through this current pregnancy but I'm really not sure I want to tandem nurse again, yet ds is only 18 months old, SO many emotions! I've been wanting to get the Adventures in Tandem Nursing Book.
Oh, my ds was 9 lbs at birth so he got what he needed even with big sis nursing!
Just wanted to let you all know that I started bleeding on Thursday and went to my OB. He said my HCG levels were too low on MOnday (25). They thought I was 3-4 weeks and I thought I was 5-6. My doc said I probably was not pregnant by the time I had the positive test - it probably never implanted. Thursday my level was 10. I am sad, but I know it my heart I really was not ready to do this again so fast - maybe ever. I was scared out of my mind. Maybe I will feel different in a few years. I feel like a failure.