German Marianne Bordt, 72, is accused of killing Camden Hiers by holding him under the water.
She then allegedly tried to commit suicide, but failed and was found by her husband.
Bordt was about to stand trial in Florida but it has been delayed after the revelation about her head injury.
The prosecution want more time to prepare a rebuttal against the last-minute defence witness - a German-born psycho-neurologist, local paper the Apalach Times reported.
Manfred Grieffenstein had carried out a series of tests on the 72-year-old and was expected to tell the Florida judge that she has mental problems which date back to an injury she suffered during World War II, the paper said.
He is expected to argue that Bordt suffered a head injury in a bombing raid by Russian forces on her hometown of Breslau on October 7, 1944.
This, combined with symptoms of depression and paranoia contributed to the crime, he will argue.
But the prosecution say they need time to find an expert in dementia and memory loss who could contradict Grieffenstein's argument, so the judge opted to postpone Monday's trial until both parties were ready.
The defence are pleading insanity and think she should be detained in a Florida mental hospital for the rest of her life. Prosecutors have called for the death penalty.