After three back-to-back miscarriages, Brittany Gould said she turned to Theranos Inc. to know if her latest pregnancy was on track.

Then, one of the company’s trademark finger-prick tests indicated she was losing another baby, Ms. Gould said. The Mesa, Ariz., medical assistant recalled dreading the moment when she would have to tell her 7-year-old daughter, who was waiting for a sibling.

“Mommy is not having a baby,” Ms. Gould said she told her.

Like those of other patients slated as potential witnesses in the criminal trial of Theranos founder Elizabeth Holmes, Ms. Gould’s test was wrong. Prosecutors have accused Ms. Holmes of defrauding patients and investors by falsely claiming her invention could accurately perform lab tests on just a few drops of blood.