A lot must have to go right in staging a successful play, from choosing a good script to managing actors’ egos and avoiding freak set-building accidents.

There are many ways even the most experienced director’s plans can unravel before rehearsals start.

But what do you do when your big name draw unexpectedly falls pregnant?

Rewrite the script – just as playwright and Hollywood director Neil LaBute has done for Diary of a Call Girl star and former Doctor Who sidekick, Billie Piper.

The former child singer, who shot to fame as a 15-year-old with the number one single Because We Want To in 1998, is starring in Reasons To Be Pretty, which opened this week at the Almeida Theatre in Almeida Street, Islington.

The drama focuses on the lives of two troubled couples examining the perception of beauty and whether being conventionally attractive is any better than being considered ugly.

Directed by Almeida artistic chief Michael Attenborough, son of Richard, Lord Attenborough, it is the UK premiere of the third and final chapter in a trilogy about physical appearance by LaBute.

Attenborough has taken the helm after the writer, who has directed Nicholas Cage and Aaron Eckhart in films such as The Wicker Man and In The Company of Men, dropped out due to Hollywood commitments.

And when Piper realised she was expecting her second child, she offered to step aside too.

Attenborough said: “She was upset and told me she was pregnant.

“However, she was Neil LaBute’s and my first choice for the part – and fortunately the character is pregnant – so with a bit of judicious re-writing we’re all delighted she’ll be with us,” he adds.

Although her character Carly is expecting in the original version, the audience is not aware until late in the play – but Piper will be six months pregnant when the run finishes in January and hardly able to hide it.

Her fellow cast members, no doubt hoping nothing in the next couple of months calls for another hasty rewrite, are Sian Brooke, whose TV credits include Silk, New Tricks and Doc Martin; Tom Burke, of Great Expectations and State of Play, and Kieran Bew, who has a string of appearances in TV crime shows such as Waking the Dead and Silent Witness to his name.

* Reasons to be Pretty is at the Almeida Theatre until January 14. For tickets and more information, visit www.almeida.co.uk or call 020 7359 4404.