Being born with additional digits - or being a "polydactyl" - is not wholly uncommon, but it is unusual to see the condition on every extremity.
The Bay Area hospital said staff did not notice the extra digits on ultrasound scans - and did not even spot it when Kamani Hubbard was born.
It was his father, Kris, who realised his son had some unusual features.
Polydactylism is genetic and the father said there was a family history of the condition. However he added his son's case was unique.
"Some family members have had, not completely developed. But not the toes."
A paediatrician at St Luke's hospital, Dr Michael Treece, said: "It's merely an interesting and beautiful variation rather than a worrisome thing.
"Imagine what sort of a pianist a 12-fingered person would be. Imagine what sort of flamenco guitarist. Think of their typing skills."
Others have pointed out that his family may have some difficulty establishing what the extra pig does in the famous 'This Little Piggy' refrain.
Famous polydactyls have reputedly included the English music hall entertainer Little Tich and Anne Boleyn - although the latter is still disputed by historians.