EXCLUSIVE: Jennifer Aniston reveals her 'plump new pout' as her 40th birthday looms

So Jennifer Aniston must be feeling the pressure to look her best.

The former Friends actress has, it appears, made some slight adjustments to her famous face. 

Puckering up: Jennifer Aniston's relatively thin top lip in 2005 (left) now has a bee-stung look

She seems to have had her lips 're-engineered' with a series of plumping injections.

The work apparently took place in early July, after Miss Aniston ducked out of sight in the weeks before the birth of her ex-husband Brad Pitt's twins with Angelina Jolie.

'It seems obvious she had some work done,' a close friend said, noting her lips had a 'stiff, unnatural pucker that just didn't fit the rest of her face'.

Lip service: Jennifer in Hollywood yesterday

Earlier this year, the actress started dating rocker John Mayer, 31, following a failed relationship with actor Vince Vaughn and the demise of her five-year marriage to Pitt.

According to U.S. reports, she has set her heart on marrying Mayer and having a baby with him.

'There's a lot of pressure on Jennifer dating John,' said one friend. 'Every other girlfriend he has had has been ten years younger than Jen, and she's very competitive about keeping up.'  

Younger lover: Jennifer is currently dating singer John Mayer, who is almost a decade her junior

Miss Aniston, who has admitted to having surgery on her nose, is also said to have embraced Botox, to smooth wrinkles from her cheeks and forehead.

The friend said: 'She's obsessing about her face because it's sort of been the only part of her that's aged.

'From the neck down she's like an Olympic athlete, with great proportion, tone and skin colour.

'But she worries a lot about wrinkles and her thinning lips. That's what she focuses on.'