My dd weight gain also slowed down at 4 months at around 400g but then 700g in 5 month. As long as your bb's weight gain is maintain at the same quartile of the weight, that should be fine. I think sometimes they grow more and sometimes they grow less.
I have an extra new set of math card ( follow the Glen Doman recommended size), printed in China. The quality is pretty good. This web sells for s$60 (Right Brain Education - mumsbuy.com)
Random Red Dots Flash Cards
S/N: RBF02A
Description:
Suitable for 3 month - 2 year old
Flash card...
