By around 2 months, most babies:
- Smile as a way to engage others.
By 4 months, most babies:
- Start using their arms with purpose. For example, babies may move their arms and squirm when excited. Or they may "swipe" at dangling objects.
By 6 months, most babies:
- Have doubled their birth weight.
- Are able to sit with little or no support.
By 9 months, most babies:
- Get upset when you or another caregiver leaves.
- May have started to crawl.
By 12 months, most babies:
- Have tripled their birth weight.
- Are expressive and have formed a close attachment to their parents.
- Understand some words and start to figure out the meaning of many others.
- May be able to say a few words.
- May be walking.
Premature infants typically reach milestones later than others of the same age. But they are usually on schedule for their expected time of birth. For example, a baby born 2 months early might reach milestones 2 months later than a full-term baby born at the same time.
Healthy babies who were born prematurely usually reach normal developmental levels for their age by the time they are about 24 months of age. Learning and thinking skills usually are first to catch up. Motor skills are often the last to catch up.