I nursed during my second and third pregnancies. No problem with either pregnancy. My milk dropped off a lot, and the older child self weaned by the 6th month of the pregnancy. I've read that some women make more milk, and some toddlers are more determined to keep nursing, and keep going all the way tgrough, but that hasn't happened for me yet.
Yes you can nurse while pregnant. Many women do it.
I wasn't able to nurse after I got pregnant - my supply took too much of a hit. Make sure you are in contact with your medical folks - baby's pediatrician to make sure that your baby is getting what it needs from your milk still and your OBGYN to make sure the baby you are pregnant with is getting what it needs while nursing.
I wasn't able to nurse while pregnant. My supply took too big of a hit. I have heard advise against it and have heard others were able to do it successfully. Be sure you are having open conversations with your pediatrician as well as your OBGYN.