According to the Daily Mail, the 31-year-old was due to return to work from maternity leave last month, after she and former colleague Andrew Hepburn welcomed their first child in January 2011.
However, Siadatan is now said to have 'amicably' left her position after informing her bosses that she is expecting her second baby with Hepburn.
She was the last of the show's first six winners to remain employed by Lord Sugar, who has offered a business investment as its prize since last year.
A source told the Mail: "Yasmina never even came back to work. She handed in her notice recently and said that she was pregnant again. Yasmina had a great future at the company and Lord Sugar had great faith in her, but she is concentrating on her family."
A spokesman for the business tycoon said: "When she became pregnant again, Yasmina took the decision to leave the company but remains on good terms with everyone at Amscreen.
"Of course, both sides would have liked to work with each other for longer but Lord Sugar knows that family must come first.
"Yasmina did a great job during her two years in the healthcare division of Amscreen and the door is always open should she ever want to return."
The eighth series of The Apprentice begns next month on BBC One.