'Dr. 90210' Robert Rey -- Now Making Bigger Boobs ... Using Stem Cells

Dr. Robert Rey is now killing two birds with one scalpel, if you will -- giving his patients bigger breasts without implants, and instead using unwanted fat and stem cells to get the job done.

The "Dr. 90210" star says he's doing implant surgery less and less these days -- and now up to 85% of his patients are opting for the stem cell breakthrough to naturally boost their racks.

It's pretty damn amazing ... watch the clip -- Dr. Rey explains how it works, and why he thinks it's better.

BTW ... this got us thinking about Paris Hilton -- who posted a revealing cleavage shot this week, but won't say if she had a boob job. God knows she could afford a few stem cells ... we just don't know where she'd get the unwanted fat.