Fish patterns

This task is about patterns and their rules.
The pattern of spots on Kevin's fish changed each week as it grew.

Week 1

Week 2

Week 3

The pattern on Kevin's fish kept changing like this. Using the pattern, draw what the fish would most probably look like at Week 4, Week 5, and Week 6.

a) Week 4 
b) Week 5
c) Week 6 
d)
Describe the rule you used to work out the patterns on Kevin's fish.