The measurements are from the top shoulder to the bottom snap buttons and from armpit to armpit (A-A)
0-3M 40cm (15.75") L, 24cm (9.45") A-A
3-6M 41.5cm (16.34") L, 25cm (9.84") A-A
6-12M 42.5cm 916.73") L, 26.5cm (10.43") A-A
12-18M 45cm (17.72") L, 28cm 11.02") A-A