Depending on when they can stand, the most challenging age to lift your child is between 2 and 3 months. Why? Because kids can turn themselves in bed, you need to put 5 cm between the top of the bed and the mattress for safety concerns, which means more leaning down.
And because you’ve had a few difficult months with short nights and the child is no longer a baby, it starts to weigh something, and you see, this is when everything comes together to kill your back, so do your best. That’s the best posture. See the back pain of new mums here.