5 to 7 months
The first time baby sits up, it'll probably be with your help. A young baby may not be strong enough yet to keep her chest raised, let alone keep her balance, so she'll likely slump or tip over. But your baby will get better at holding herself up as she approaches the middle of her first year; you'll probably see her hold her hands out in front to steady herself. As she becomes more comfortable, she'll be able to do other things at the same time -- like reach for a toy. Eventually she'll figure out how to pull up from her stomach into a sitting position and then lie down again.