I feel stupid for even asking:) I have two female chihuahua's. One (Gigi) is 14 months old and the other(Rosa) is 6 months old. The six month old has no signs of having any nipples. I never noticed this on the older dog. Even before she went into her first heat you could see tiny nipples. Huh????
Take her to a vet and have her checked out. I have never heard of a dog not having any nipples. Could be they are just tiny and blend in with her coloring. She is old enough to be spayed and maybe when you have her checked out, you can set up an appointment to have it done. If its true that she really does not have nipples, you certainly do not want to risk a pregnancy.
"No Matter how little money and how few possessions, you own, having a dog makes you rich." - Louis Sabin