The Duke and Duchess of Cambridge are expected to become parents in July, the royal family said.
Kate was expecting just one baby. But there have been speculation that she might be carrying twins.
Kate earlier fell ill in December when she became severely dehydrated. She was admitted to the King Edward VII Hospital in London and doctors diagnosed her with hyperemesis gravidarum, an acute form of pregnancy-related sickness.
The Duchess of Cambridge, Kate Middleton made a visit to Hope House, in London on February 19, 2013. As patron of Action on Addiction, the Duchess was visiting Hope House, a safe, secure place for women to recover from substance dependence.
An Italian magazine recently ignored the protests of the royal family to publish pictures of Prince William's pregnant wife Kate in a blue bikini on a private island.
The Duchess has finally begun to show her belly bump.
The British media have refrained from publishing the pictures of Kate in a bikini as they fight stricter regulations recommended by a judge-led inquiry following a phone-hacking scandal centred on Rupert Murdoch's tabloids.
Queen Elizabeth's surgeon-gynaecologist Marcus Setchell is monitoring Kate's pregnancy.
While magazines and websites in Europe have printed long-lens shots of the couple, Britain's tabloid newspapers have so far held back.
The publication of topless pictures last year against the couple's wishes was condemned by British Prime Minister David Cameron. It stirred memories of the world media's relentless pursuit of William's mother, Princess Diana who died in a car crash in Paris in 1997 as she was being chased by paparazzi.