I was so glad to see the responses from all of these breastfeeding mamas who are doing the extended and tandem nursing, because sometimes I feel like I am the only one. I nursed my oldest throughout my second pregnancy and now I am tandem nursing, although I have tried to cut my #1 back somewhat. He would nurse all the time, if I would let him, while the baby has to sometimes be persuaded to eat and when she's done, she's done.
I deliberately decided not to wean my first-born when I found out I was pregnant with #2 because I didn't want him to feel that the baby was taking his place and I thought this would be a way for us to keep our close special bond and cut down on jealousy. I'm not sure it was the right decision because now I don't know how I'm ever going to wean him.
My OB/GYN was opposed to it because she had concerns that it could lead to premature labor. There is a very good book called "Adventures in Tandem Nursing: Breastfeeding During Pregnancy and Beyond" that quelled this fear. I'm surprised no one mentioned this book. I checked it out of my local library. Lots and lots of women have nursed during pregnancy without incident. Of the many many women interviewed for the research, not a single one (as I recall) had premature labor from nursing. Those who felt premature contractions merely stopped the child from nursing and the contractions went away. Anyway, I just decided to keep it to myself after that and didn't discuss it further with my doctor.
One thing the book mentions is a natural aversion that pregnant women seem to develop to having the older child nursing while they are pregnant. Almost like a nails on a chalkboard discomfort. In animals, the mother will literally push the pup away when they try to nurse. I did feel some of this aversion and I found the nursing more painful than before, but I had made up my mind to do it and it wasn't that bad.
My milk dried up during the pregnancy, but this did not deter my oldest at all. He just kept right on suckling for comfort. Now I am pain-free and one advantage is that my milk flows like a river from all the nursing. Good luck to you if you decide to do it and you will not be alone!