When I was researching when to spay, the best advice I read was to wait until the pup is at least 6 months old..I waited until Miya was 7 months. In my opinion, and my vets, 5 months is too early to spay. Also, I was told that these little dogs go in to heat a little later. Maybe some others can offer insight on to how old their chi was when first heat came around. I didn't want her to have her first heat because I was clueless how to deal with a dog in heat. If you're capable of handling a heat and are educated about it, it might be an option, just make sure you're equipped to handle it.