I dont think you can go by that really..sometimes its right,but sometimes its not..youīll just have to wait and see how big your baby will be

.Lady was 7 weeks old and weighed in at 8ounces when I got her,and now at 14months old,she is at 2lbīs 4ounces.
Jessie weighed 2lbīs at 8 weeks old and now weighs 8lbīs at 9 months old.You can also triple their weight at 8 weeks to see how much they will weigh..but I wouldent swear by it