This is a pair of leggings knit flat in halves from waist down.

Newborn size:
Length - 31cm/12.5in
Materials - 2 x 50g balls

3 month size:
Length - 36cm/14.5in
Materials - 2 x 50g balls

6 month size:
Length - 43cm/17in
Materials - 3 x 50g balls

9 month size:
Length - 50cm/20in
Materials - 3 x 50g balls
12 month size:
Length - 56cm/22.5in
Materials - 4 x 50g balls