Tantum, 33, said "outdated but still prevalent stereotypes" of women persist at the law firm Travers Smith, which failed to extend her contract after a training period because she announced her pregnancy.
She was one of three "trainee solicitors" who weren't retained by the firm, The Daily Telegraph reported Friday.
The firm denies the allegations and said Tantum's contract wasn't renewed because she failed to show "intellectual rigor" on the job.
Tantum -- a graduate of Cambridge University who received distinctions in her law course and positive feedback at work -- said, "I felt it hard to believe I would not have secured a post had I not been pregnant."
A judgment in the hearing will be made at a later date, the newspaper said.