X-Files star Gillian Anderson is pregnant with her third child just before her 40th birthday, it emerged today.

Anderson and boyfriend Mark Griffiths already have a son, Oscar, who was born in November 2006.

Anderson also has a 13-year-old daughter, Piper, from a previous marriage.  U.S. reports revealed yesterday that the actress - soon to appear alongside Simon Pegg in 'How To Lose Friends and Alienate People' - is expecting again.

X Files star Gillian Anderson is expecting her third child

The birth expected to take place when the actress is 40. Her landmark birthday just two months away.

The news could explain her struggles reprising her role as Dana Scully in 'The X-Files: I Want to Believe', a new movie due out July 25.

Anderson, 39, admitted she found working on the action movie a gruelling task - as she is not as fit as her co-star David Duchovny.  So she left him to do majority of the stunts.

She said:'"It was hard work. I'm too old for this. David's in the best shape of his life and did most of the work. He's climbing and jumping off cliffs. 

'I was more exhausted by the end of it than he was. I'm a wimp.'

The X-Files TV show officially ended in 2002 but the actress has revived her character F.B.I. Special Agent Dana Scully for the series' second full-length feature film.

Special Agents Mulder and Scully are reunited on the big screen after six years apart.

The X-Files: I Want To Believe brings back actors David Duchovny and Gillian Anderson, following on from the last TV episode in 2002.

And sneak peak pictures of the upcoming movie, continue to tease viewers with Mulder and Scully's will-they-won't-they romance, hinting the couple may be getting close again.