I was told that as long as you don't feel any pain or strain you can lift carefully. Most likely your 2 year old is right about the limit where you feel strain so I would probably limit the times that you have to pick him up, especially as you get further into your pregnancy. Also, after you have your second one you will probably be instructed not to lift anything heavier than your newborn for a few weeks. Maybe the next few months you can work with your 2 year old to become more independent and "big boy" so that the transition with the newborn will be less of shock to him. Supervise him as he goes up and down stairs, start changing him on the floor or a couch rather than a changing table, have him crawl up on his booster seat, etc. Best of luck!