You should also take into account his or her adjusted developmental age as well.
Understanding developmental age
Your baby's birth age (chronological age) is obviously determined from the day of his or her birth. However for premature babies, it's often helpful to use an adjusted age instead. With adjusted age, the age of the baby is determined by the child's due date. Healthcare providers and parents often use the adjusted age when evaluating a baby's growth and development.
For example, a baby whose chronological age is four months, but was born six weeks early, isn't expected to be at the same developmental point as a four-month-old who was born on his due date.
The majority of premature babies catch up developmentally to their peers by the time they are three years old. However, some extremely small babies may take a little longer to catch up in height and weight.
After age three, any differences in development or size are usually due to individual differences or genetics rather than stemming from premature birth.
How old is he?
Some moms of premature babies have difficulty answering the question, "How old is he?" When a friend or relative asks, it's perfectly acceptable to state his chronological age. After all, people close to you likely know your child was born prematurely. However when a stranger asks, it becomes awkward. Unless you are comfortable explaining about your child's history, you can just give your baby's adjusted age and leave it at that.
It's just a number
Age is just a number when it comes to babies. A premature four-month-old can be more advanced in some areas than a six-month-old that was born on time. However, it's a good idea to keep your child's adjusted developmental age in mind when it comes to your expectations of his or her development. For example, don't be frustrated if your 12-month-old isn't walking. Many children do not take their first steps until they are several months past their first birthday. So in the case of a premature baby, you need to tone down your expectations and celebrate small milestones whenever they come, instead of being consumed with what the "average" child is accomplishing. If you feel your baby is becoming significantly behind in development or growth, talk to his or her pediatrician about your concerns.