Sitting is a gradual process. It starts when a baby gains the ability to hold her head steady and continues as that muscle control works its way down her back. An infant first has to get control of her neck, shoulders, lower back, and legs in order to sit up and play, using her head and hands freely without toppling over unexpectedly. By the end of this month, your baby will have begun the process: She'll probably be able to sit holding her head and shoulders up, but the middle to the bottom of her spine will still sag a bit. You should prop your baby in a sitting position often so that she can practice this new skill. Surround her with soft cushions or rolled blankets to support her back and protect her from toppling over.
Soon your baby will also be able to maneuver himself into a sitting position. Since an infant who can move around this much could easily tip over a stroller or fall out of a crib, you need to start being extra careful. Always use restraining straps when transporting your baby, and make sure the crib rail is up when he's in the crib. And of course, never leave him unattended on your bed or any other elevated surface for even a moment. It's also critical that you remove any crib toys and mobiles, which now pose a strangulation hazard.
All content here, including advice from doctors and other health professionals, should be considered as opinion only. Always seek the direct advice of your own doctor in connection with any questions or issues you may have regarding your own health or the health of others.