There's 3 week and 6 week.
Then, think about clothing sizes. There's a growth spurt at each of the generic clothing sizes. Which is: 3 months, 6 months, 9 months, 12 months, 18 months, 24 months, 3t, etc.
That's why you don't see size 8 month clothing or 5 month clothing. The generic ages refer to the general growth spurts.
Some children spurt a month prior or after, etc. but those are the common spurt months.
Hope that helps!
SAHM to Magnolia May (09/10), Luke Russett (04/13) and expecting 11/16. Wife and best friend to my airman.