Kate Davies, a sales negotiator at Anthony J Turner in Hebden Bridge, was fired four days after she told her boss she was expecting.
The tribunal decided Mrs Davies had been unfairly dismissed because she was pregnant and was discriminated against on the grounds of her sex.
The estate agents, trading as Anthony J Turner, Crossley Crosland and Uttley, could face a hefty compensation bill. After the hearing, Mrs Davies, of Byron Terrace, Sowerby Bridge, said: "I'm very pleased with the outcome. I came here for the truth and that's what I've got."
Company owner Anthony Turner said he believed Mrs Davies had been "economical with the truth" when she did not admit she was pregnant during an interview.
A week into the job, which started in January this year, she told her boss Francine Turner, wife of Mr Turner, that she was pregnant.
Mr Turner said he sacked Mrs Davies after just 10 days because she was not "up to the job". He added: "Kate Davies was dismissed when she was pregnant but she was not dismissed because she was pregnant."
Mr Turner's son, Ben Turner, admitted he had asked Mrs Davies if she had children during her first interview.
He said he and his partner were expecting a child and had not yet chosen a name. He said he was interested to hear what names other people chose for their children.
Tribunal chairwoman Christine Lee said this was not an acceptable reason to ask a potential employee about their personal circumstances.
She said: "Mr Turner believes Mrs Davies misled him into employing her by not telling him that she was pregnant.
"The implication of this is that had he known she was pregnant he wouldn't have taken her on."
The tribunal also said the estate agents acted illegally by not giving Mrs Davies holiday pay. A hearing to decide compensation will take place on October 31.