[dottiesyrky]

Perhaps a second opinion is in order? And certainly go ahead with the spay. But if you trust your vet and want him to go ahead and treat your baby, I would suggest at least a biopsy of the lumps or the even remove them if that is a relatively easy thing to do. I say this as my neighbor had a girl dog who had mammary lumps and the vet said they were cancerous but not aggressive, so he removed them and is going to monitor the dog in the future. The dog is doing so well now. This dog was rescued some time ago and had had a couple of litters. It was spayed before adoption.
Good luck, whatever you decide to do, and please update on your baby.