I have a 7 week old baby, one of his pupils is bigger than the other is there a reason for this?

Could be serious: In small babies, asymmetric pupils should raise concern. If any other symptoms are present, esp neuro symptoms like sleepiness, acute onset, or with fever, urgent evaluation would be recommended. If present since birth in an otherwise apparently normal child, still would warrant evaluation by the pediatrician for underlying disorder; there are too many to detail in this space, of varying severity.

Have it checked: Could be physiologic anisocoria (born that way), but should also have other medical conditions ruled out and the pupil formally measured to quantify size difference and to check reactivity in light and dark.

Get evaluated: There are a number of possible causes for this. Some are harmless, but some are more concerning. You should see a pediatric ophthalmologist to get evaluated.

YES,: You need to see a local ophthalmologist (not to be confused with an optometrist) as soon as possible. Although this could be a benign incidental finding, other more worrisome diagnoses are a concern. Although not warranting an emergent trip to the er, your child needs to be evaluated as soon as possible.