Ruth Negga has been thinking about starting a family with her boyfriend Dominic Cooper - because she knows her biological clock is ticking.

The 35-year-old actress has been dating the 39-year-old actor since 2009 and Ruth admits her thoughts often turn to babies now they have been in a serious relationship for so long.

Speaking to Net-A-Porter's The Edit magazine about her long-term relationship, the 'Loving' star said: ''By the time you're in a great relationship, it's like, 'Oh, I'm 35. S**t.' So there is part of your thinking that is, 'Do I want kids? When? If? How?' But it's there, all the time, preoccupying me. And that's not being an actor, it's being a woman.''

Despite society making huge leaps forward when it comes to women's rights and maternity leave, Ruth doesn't believe there are any jobs that are fully compatible with motherhood.

She said: ''I don't know what [job] really creates a safe space for a woman to have time out, prolong the human race, and return to work in a very supportive way.''

Ruth has enjoyed a seven-year long romance with 'Mamma Mia!' star Dominic, but before finding love with him she was warned not to date an actor.

She said: ''People say, 'Never go out with an actor.' But if you were with someone with a normal job, and one of you had to go away, how would that work?''

However, Ruth not only dates a fellow film star but she also co-stars in the American television show 'Preacher' alongside her partner.

But she has no qualms about working with her boyfriend and would, in fact, feel ''lonely'' if Dominic wasn't working on the same series.

She said: ''It would be so lonely if [Dominic] wasn't there.''