Shalom! I would like to know why chelev of cows is not permissible but if the cow has a new born - the chelev of the new born is permissible. Thanks

Chelev of animals is forbidden whatever their age. What is permitted is the chelev of an unborn calf found in the uterus of a slaughtered cow. This because the calf does not have an independent status of its own (Shulchan Aruch Yoreh Deah 64 2).