either of two large organs lying within the chest cavity on either side of the heart; they supply the blood with oxygen inhaled from the outside air and dispose of waste carbon dioxide in the exhaled air, as a part of the process known as respiration.
Most babies first pull themselves up to a standing position between about eight months and 10 months. At this stage your baby will need to hang on to you or a solid piece of furniture. By 11 months she may be able to stand on her own.
One of the first developmental milestones that new parents look forward to is the day their baby learns to sit up. Most babies will learn to sit by themselves somewhere between 6 and 8 months, however, it can happen as early as 4 months or as late as 10 months with no cause for alarm. As soon as your baby can support and control her head and has mastered rolling from back to belly, she can start learning to sit.