Should i be worried if my baby's size has not changes in five weeks?

Yes: Babies are supposed to have growth that is measurable by 3 weeks at least. If your baby isn't growing there is a problem with the blood supply through the placenta. Babies that aren't growing well need special monitoring to make sure that they are doing well enough to stay inside. Sometimes they will have to be delivered early to avoid problems like stillbirth.

Not to worry: Most babies have to first try sucking the bottle or breast. This may take a while. However, as long as they look healthy and are drinking the milk, i think the little one is doing well if he or she is takingin the milk. For more concerns you may want to call your pediatrician.