The stages of infant development can be split into four age stages, each lasting three months. During these stages, there are general milestones that you can expect him to reach. These are general guidelines that allow you to ensure your baby is developing correctly, but it's important to remember that not all infants reach these at exactly the same age.
In the first three months, your newborn baby will quickly get to grips with her new environment. During this time, she will start trying to lift her head, kick her legs and use her hands to grip. Watch out for that first little smile during this stage. She will watch you intently as you speak to her and if you make "baby noises" she will try to copy. Babies at this age also learn to recognize familiar people and she may even stop crying when she hears your voice.
At 3 months, your baby will be moving into the next stage of his development. Pediatric expert Dr. William Sears says most babies learn to roll during this stage as they become stronger in their upper bodies. He will realize he has control of his own hands and start to use them to deliberately touch and grab things -- including your jewelry! Smiles will be common now and you will be able to make him laugh. His interaction with you will become more developed and when you talk to him he will make noises back that sound like a little conversation. He will also enjoy a simple game like peek-a-boo.
At some point between 6 and 9 months, your baby is likely to learn to sit without support, although you will need to be aware of what is behind her in case of any wobbles. PBS The Whole Child says this is usually the time that babies learn to crawl, although some will find other ways of moving such as sliding on their tummy or bottom. You can encourage her to achieve this milestone by placing toys just out of reach. She will start to tell the difference between friends and strangers and may cry at people she does not recognize. Sounds like "bye bye" will become familiar as well as her name.
Your baby will probably be able to sit confidently now and get himself into this position without help. Healthy Children says he should be able to support his weight on his legs, even if holding onto something. He will be thinking about walking and may even do this while holding on to furniture. He will master the art of throwing during this stage and will enjoy dropping and picking objects up. He will start to realize that an object still exists when he can't see it and will look for hidden toys. He will understand some words and may even start trying to say a few himself.