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.
By the time your baby is eight months old, she should be able to sit well without support (NHS 2013, Sheridan 2008). Nearly all babies can sit on their own by the time they are nine months old (NHS 2013, Sheridan 2008). How will my baby learn to sit up? While you can prop your baby in a sitting position almost from day one, sitting independently doesn't begin until your baby has head control.