Kate Middleton who is pregnant with her second child, is reportedly expecting twin girls.

According to Star magazine's 20 October issue, the Duchess of Cambridge and Prince William found out the news during a recent ultrasound.

"When they looked at the screen and saw two tiny human beings, they were completely overwhelmed," a source close to the couple told the magazine, according to Hollywood Life.

"Will said Kate's mouth dropped open and her eyes almost popped out of her head, then she squealed, 'Oh my God!' He'll never forget the look on her face — it must have been priceless!" the insider added.

"William phoned his father straightaway to give him the good news," the source revealed.

Last month, Clarence House confirmed that Kate is pregnant with the couple's second child and she is suffering from hyperemesis gravidarum, a potentially serious illness causing vomiting and a bodily reaction against intake of food and liquids.

People recently reported that Kate has been staying with her parents, Carole and Michael Middleton, in Bucklebury.

Kate is "not at all well, the poor thing," a family friend of the Middletons told the website.

"When you're ill, the place you feel most comfortable is at home with your mum, isn't it?" the source added.

"When she's there, Kate can just relax and sit on the sofa and not have to worry about having to deal with all the staff and everything else that comes with a big palace full of rooms," the insider said.

When Kate was pregnant with Prince George, she was hospitalised with acute morning sickness at the King Edward VII Hospital.

She gave birth to Prince George at the St Mary's Hospital, London, on 22 July.