Baby SharkBaby Girls' 100% Combed Cotton Underwear Panties in Sizes 18m, 2/3t, 4t, 4, 6, 8

How true to size are these? My daughter is 2, she weighs 32lbs and wears 3t in clothes but according to this chart she would need a 4t.

What size is 18? My kids are in 5T.

It says size 18. Does it mean 18 months or 18 in big kid size ?