The woman, who became mother to octuplets on January 26, already has six kids - including twins.
All her previous children are living with their grandparents while she is recovering in hospital after delivering six boys and two girls.
While the octuplets are doing well, the identity of the mum, believed to be Hispanic, is being kept secret.
A spokeswoman at the Kaiser Permanente Bellflower Medical Center, California, said that the mum of 14 is recovering well.
The revelation that the mother already has six children will intensify speculation that she has had fertility treatment.
Also, the news will spark questions about why a mother of six would receive help to conceive again.
"If a medical practitioner had anything to do with it there's some degree of inappropriate medical therapy there," The Sun quoted Michael Tucker, a leading US researcher in infertility treatment, as saying.
The babies were born nine weeks early and are believed to be only the second set of live octuplets born in America.