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.
Q: When will my baby start sitting up? A: Your baby will probably master the skill of sitting up unassisted between 5 and 6 months. It's a slow process -- in order to sit up, your baby has to tackle a number of physical feats, including holding his neck steady and having control over his shoulders, lower back, and legs.