Rumors about pregnancy of Kate Middleton started doing the rounds when the Princess covered her midsection during a dinner event in London. The news came out when Kate was diagnosed with hyperemesis gravidarum (a form of morning sickness) after falling sick at her family home. This form of morning sickness affects only one out of fifty women and is known to affect women who are expecting twins.
Later, Prince William was seen leaving King Edward VII hospital in London and whisked away in his car without commenting on any questions that reporters present there asked from the Prince.
According to British papers, this news was only told by Prince William to Queen, his dad (Prince Charles) and his brother (Prince Harry) through e-mail as he is serving in Afghanistan.
Source: NY Daily News