The singer posted the photo on her Twitter account with the caption: ‘Happy 1st Day of Summer! Keep those lips hydrated! @eosproducts #mint #summersolstice.’

The photo showed the 33-year-old applying some Eos mint lip balm as she covered her bump in a tight, blue, jersey dress.

Christina is expecting her second child, her first with fiancé Matthew Rutler. She met Matthew on the set of her 2010 movie Burlesque.

The talent show judge is also mum to 6-year-old son Max from her marriage to music marketing executive Jordan Bratman, who she divorced in October 2010.

Speaking about her current pregnancy, Christina told an American magazine: ‘I’m really excited to meet our baby girl...This has been a very easy and enjoyable pregnancy.’

Reports of Christina’s pregnancy emerged just weeks before she announced her engagement via her Instagram account, showing off her engagement ring in a ‘hand selfie.’

Christina is set to take some time of from her judging duties on American talent contest The Voice, where her seat will be kept warm by No Doubt singer Gwen Stefani, who also recently gave birth.