If you had a c-section, you can start lifting weights after 8-12 weeks. A C-section is major abdominal surgery and often requires a long recovery
If you lifted weights prior to giving birth, then you won’t need to wait as long. One of my good friends lifted heavy weights up until she was 39 weeks pregnant.
Some women find that they recover more easily from their second pregnancy, but only if you have an uncomplicated delivery.