Jennifer Aniston is reportedly ''desperate'' to have a baby.

The 'We're the Millers' star - who is married Justin Theroux - has been trying to get pregnant since their engagement in August 2012, and she is said to be frustrated that she's not expecting yet.

A source told MailOnline: ''It's starting to be a source of irritation for Jen ... [She] purposely left her work slate clean for the next few months to prepare for motherhood.

''She puts a brave face on the rumours, but it masks her desperate desire to be a mother.''

It is also said the couple's wedding may be delayed due to their working commitments, but it is claimed Jennifer is ''understanding'', despite the postponement raising questions from their friends.

The insider added: ''Jen's completely understanding of Justin's opportunity to elevate his career to another level and prepared to wait though friends are starting to question whether they'll never make it down the aisle!''

The former 'Friends' actress has recently spoken about their future wedding, insisting they haven't set a date yet because they want to wait until the timing is ''perfect''.

She previously said: ''We just want to do it when it's perfect, and we're not rushed, and no one is rushing from a job or rushing to a job.

''And, you know, we already feel married. There have been no cancelled weddings. There have been no postponed weddings.

''There have been no arguments about where to get married. Just clearing all that up.''