Britain's Prince William is expected to buy Duchess Catherine a ruby birthstone after she gives birth.

Marcia Moody, author of 'Kate: A Biography', believes William has already eyed up the perfect push present - a gift traditionally given to a mother from her partner after she has a baby - for the duchess, known as Kate Middleton before her nuptials, and thinks he will opt for a special gem.

Speaking to this week's Bella magazine, which is out now, she said: "Like lots of new dads, I'm sure William will buy Kate a present once the baby is born, probably jewellery and something romantic.

"Not many people have noticed, but one of the rings she's been wearing a lot recently has alternate garnets and pearls.

"They're his and her birthstones - Kate's is garnet [for January] and William's is pearl [for June].

"I feel there's only one person that would give someone that - William.

"I think it's likely he'll get her something with the baby's birthstone. [The birthstone for July is a ruby.]"

Should William purchase a plush push present for Catherine - who is due to give birth on July 13 - then it wouldn't be the first time he has gone to such great romantic lengths.

Marcia added: "William's very romantic. Romance is really just thoughtfulness and he includes Kate a lot.

"On a tour of Canada, they rowed out to the little island near Yellowknife for a picnic - apparently that was his idea. It was just the two of them.

"And the £1,900 green amethyst and diamond earrings he got her for Christmas in 2011, he probably would have chosen them himself."

This week's Bella magazine is out now.