By Dave Madden

CAMDEN, N.J. (CBS) – A formal arraignment is set in Camden tomorrow for a woman charged with the murder of her son more than 8 months ago.

In late March, 24 year old Lydia Vasquez brought her 19 month old son, Matthew Thompson, into the hospital saying he had fallen down a flight of steps. The child died two weeks later, and investigators didn’t buy the mother’s story. But Jason Laughlin with the Camden County Prosecutor’s Office says it took exhaustive tests to conclusively prove it.

“It took a matter of months to get the results of those tests, look through them, analyze them, bring them to experts to say ‘we’re very confident that this is what happened.’”

The answer, shaken baby syndrome came in last week, leading to the arrest of Vasquez on a charge of murder. She’s now being held on $500,000 bail.