I am 6 weeks and my doctor advised me not to lift over 35lbs. This is simply because it can weaken your pelvic floor and predispose you to preeclampsia! I would definitely check with your OB!
I am 8 weeks, my son is about 45lbs, and 5 next week. Sometimes, it can't be helped, when they are sick or fall. A lot of it depends on the individual, so your Dr can tell you for sure, but to never lift a younger one ever is unrealistic unless you are already high risk or having complications.
I am still doing crossfit 3-4 times per week... lifting more than that.... I think you just need to be careful how you are lifting items so you don't stress out your body.
Re: Lifting