It's a boy:

[link to www.mommyish.com]

Kate Middleton Bought A Blue Bugaboo, So Obviously They’re Having A Boy.

But Kate Middleton has confessed to buying a blue Bugaboo so she’s definitely having a boy, right?

[link to www.dailymail.co.uk]

Kate's biggest clue to the sex of her unborn child... she buys Bugaboo buggy in baby blue

Duchess purchased a blue Bugaboo buggy - a celebrity favourite
She has said she would prefer a son while William wants a daughter
If she does have a son, George is one of their favourite boys names