The 38-year-old Australian actress, who is best known for her roles as Roberta Williams in Underbelly and Billie Proudman in Offspring, said that she hadn't given children much thought until recently.
Stewart told The Daily Telegraph: "[Motherhood is] something I definitely want to do. I do love children. In five years' time, it would be nice to have a kid or two.
"I have a ridiculously adorable group of nieces and nephews. Being an aunty is good [but] the reality is that the window shuts at some point. Any woman over 35 is conscious of that."
Stewart also revealed that she "loves" her role in Offspring, saying: "The show gets better and better as time goes on. It's a bit left-of-centre and can be fabulously odd.
"I've had a run of characters that have been outspoken and badly behaved, and I relish that. It means I have permission to do whatever I want and say the things we all think but can't normally say."
She admitted that her parents were "secretly horrified" when she said she wanted to pursue acting as a career, but said that they've been "amazingly supportive".
Stewart also said that she doesn't enjoy the "fame and celebrity" that goes with the job.
It was recently announced that Offspring will return to Australian television screens for a third season next year.
The show was nominated for eight Logie awards earlier this year, including 'Most Popular Drama' and 'Most Popular Actress' for Stewart and her co-star Asher Keddie.
Keddie has previously said that she loves Offspring because it's "very eccentric" and "unpredictable", while Stewart revealed that she is bonding well with her fellow cast members on set.
Offspring airs on Mondays at 8.30pm on Network Ten.