Most singers lose weight before they go on tour owing to all the rehearsals and training they have to put themselves through.

However Britney Spears appears to have gained weight, yet again, in the run up to her Femme Fatale Tour in the U.S. with chunky thighs and a bloated belly.

The 29-year-old singer hit the stage at the Power Balance Pavilion in Sacramento, California in the kind of shape you might expect to see someone who is taking a career break – not embarking on a hectic, 33-date tour.

Fuller figure: Britney Spears’ midriff was far from flat during the first night of her Femme Fatale Tour in the U.S.

Hanging out: The singer has a slight muffin top as she parades around on stage in a tiny pair of shorts and bra

Prancing around on stage in a variety of skimpy outfits, it was clear that this was not the Britney some might have expected to see.

Perhaps her comfortable life with boyfriend Jason Trawick has contributed to her fuller figure, or maybe she just hasn’t had time to focus on getting in shape with her career, two sons and tour to think about.

Regardless, she hit the stage in a variety of eye-catching costumes as part of the racy new show, including a kimono-style gown, fishnet stockings and studded denim hotpants, opening with her hit track Hold It Against Me and tearing through a set-list of 22 songs before closing with Toxic and Till The World Ends.

Fluctuating: The pop star’s weight has been up and down throughout her career

The white stuff: The singer and her dancers wore white trenchcoats during another of the many energetic routines

Raunchy: Britney’s show was typically racy with lots of exposed flesh and leather-clad dancers

The outfits come courtesy of designer Zaldy Goco, who says: ‘The real inspiration of course is Britney herself. We’re creating the show for her. But there’s a creative script that was done by the director, Jamie King, and it’s a whole thing about a femme fatale, and femme fatales throughout the ages … she’s sort of changing personas throughout the show.’

She added: ‘Britney is a very 𝑠e𝑥y girl, so everything has to be 𝑠e𝑥y.’

Reviews of the show have been hugely positive, in particular the live singing.

Celebrity blogger Perez Hilton commented: ‘I really enjoyed this show, and I’m being honest. As a stand alone piece, the Femme Fatale tour is thoroughly enjoyable.

‘Britney sings live and I feel like this time the show and her are on the same level, Britney seems to have gotten her spark back.’

Skimpy: Britney’s dazzling array of outfits was designed by Zaldy Goco

Impressive: Fans and critics alike praised Britney’s opening show – in particular the live vocals

Revving up: The show was the first of 33 dates across America before she heads over to Europe

In the past Britney has been criticised for miming through many of her live shows, however, the majority of the vocals in this opening show were sung – occasionally beefed up with recorded vocals.

The opening video starts with the words ‘the game of cat and mouse’ while a man watches Britney from security monitors. He’s like a stalker trying to capture her and she’s running from the police in a leotard.

She then kicks off with Hold It Against Me, the dubstep-influenced track produced by her in-house team Dr Luke and Max Martin.

Suck it in: Perhaps Britney should have spent a little longer in the gym before embarking on her tour

Release: Britney is put in jail during a performance of Up N’ Down which was her second song during the concert

Professional: The star commanded her audience throughout

One of the highlights of the show is when she lapdances for a male fan, grabbed from the audience.

At the end of How I Roll, Britney tells her audience: ‘Sorry about the missing prop. My workers apparently don’t know how to put props on stage. Does anyone out there wanna help me find someone from the audience to help with the props?’

She then talks with a female dancer about different guys in the audience before picking one lucky fan and bringing him up on stage.

Captured: Britney dances for one lucky male fan who was grabbed from the audience

Stroke of luck: The fan looks a little intimidated as he’s surrounded by Britney and her backing dancers

On a high: Britney was buzzing after the spectacular show tweeting to fans to speak of her excitement saying: ‘Can’t sleep. WAY too much Adrenaline right now. Sacramento was on fire tonight. You ready San Jose? Cause I am!!!!’

During the course of the show the stalker reappears several times, at one point talking about femme fatales through the ages.

Full of colour and energy throughout, the show ends with Britney stripping off a kimono to unveil a flashing LED suit.

After the show she tweeted: ‘Can’t sleep. WAY too much Adrenaline right now. Sacramento was on fire tonight. You ready San Jose?  Cause I am!!!!’

The Femme Fatale show is based around an action-adventure theme, beginning with a video of the pop star being chased by police and with the set changing throughout the show to reflect the different identities Britney takes on throughout proceedings.

A visual feast: Britney sits on a swing high in the air while a dancer is suspended beneath her

Going for a ride: Britney straddles a giant guitar during the opening show of her Femme Fatale tour

Adult: Britney’s show was typically raunchy with the singer wearing lots of revealing outfits and getting steamy with her dancers

There are also costume changes to match these identities — nine in all — including an angel outfit, a kimono, and a kinky leather motorcycle-gear ensemble, specifically for her “S&M” performances.

Rap star Nicki Minaj will be injecting some colour and energy into the show’s – appearing as an opener for Britney at each of her U.S. tour dates.

She said: ‘I want to go all out and make it over the top and theatrical. There will be costume changes and hair changes. Lots of 𝑠e𝑥y outfits and dancing. I don’t know how to do things normal and boring.’

Back in black: The singer wears yet another sparkling outfit

Laid back: But her show was anything but relaxed as Britney worked through one high-energy routine after another

Support act: Rap superstar Nicki Minaj opens for Britney

Britney will perform 33 dates across the U.S. before taking the Femme Fatale show to Europe in September.

Before the show she got a good luck hug from her boyfriend Jason Trawick tweeting a photo of the two from her dressing room.

Britney was wearing a pair of white pants and bra, which she teamed with silver boots and black tights.

Break a leg: Britney Spears in her dressing room with boyfriend Jason Trawick on the opening night of her world tour last night in California