Baby's first year is filled with many developmental milestones, and parents often ask when to expect each one to occur. It's important to remember that every baby is different and many factors affect development.
Usually the first milestone parents anticipate is their baby rolling over. The time frame depends on the child's weight and sometimes his temperament or disposition. A calm baby on the heavier side may roll over later, while an active, smaller child may roll over sooner. Babies can roll over anywhere between two and six months. Because babies can roll over at such a young age, it's important to never leave them alone on high surfaces.
During their first six months, babies learn to grab onto things. If you have used a mobile or a toy bar, you may have noticed that baby isn't all that interested in it until he is around three months old. It is only at this point that he will begin to swat or reach towards objects. Your baby will usually practice this for several months before he is able to deliberately grab at something.
A baby can usually sit while supported by four months, and by six months she may be able to sit for a short time without any support. You can encourage this by holding on to your child's hands and seeing if she attempts to pull up to a sit. Many babies have this mastered by seven months, however it could happen later for larger babies. While your baby is practicing sitting up, keep soft pillows around her, and don't leave her alone. She could suffocate if she falls face first into the pillows. (See Baby sit up, both supported and unsupported.)