A photograph taken in London on Wednesday July 2, 2014, to mark Britain's Prince George's first birthday, shows Prince William (R) and Catherine, Duchess of Cambridge (L) with Prince George during a visit to the Sensational Butterflies exhibition at the Natural History Museum in London. Britain's Prince William and his wife Catherine on Monday thanked well-wishers around the world as they prepared to celebrate their son Prince George's first birthday. AFP PHOTO/John Stillwell/POOL EDITORIAL USE ONLY (Photo credit should read JOHN STILLWELL/AFP/Getty Images)
Catherine and William expecting again
01:37 - Source: CNN

Story highlights

The baby will become fourth in line to the British throne

"The Queen and members of both families are delighted with the news," the palace says

As in her first pregnancy, Catherine is suffering from acute morning sickness

British Prime Minister David Cameron says he is "delighted by the happy news"

CNN  — 

Another royal baby is on the way.

Britain’s Duke and Duchess of Cambridge are expected to become parents for a second time, Buckingham Palace announced Monday.

“The Queen and members of both families are delighted with the news,” the palace said. It didn’t say when the due date will be.

As during her first pregnancy, Catherine is suffering from hyperemesis gravidarum, the palace said. The condition involves nausea and vomiting more severe than the typical morning sickness many women suffer during early pregnancy.

She is being treated by doctors at Kensington Palace and didn’t accompany Prince William on a planned visit Monday to Oxford.

Royal baby: The new line of succession

The decision to announce the pregnancy was made when it became apparent Catherine was not well enough to go on the Oxford trip and that a number of other forthcoming engagements could be affected, a royal source told CNN.

Why does the ‘spare heir’ matter?

“The couple only found out very recently. The Duchess has not even reached the crucial 12-week stage,” the source said.

William told reporters in Oxford that the couple were thrilled and Catherine was “feeling OK.”

“It’s been a tricky few days,” he said. “We’re hoping things settle down soon and she feels better.”

The couple’s first child, Prince George, was born in July 2013.

The second child will become fourth in line to the British throne.

Prince Charles is first in line, followed by William, Prince George and then eventually the new baby. The new child will knock future uncle Prince Harry down to fifth.

British Prime Minister David Cameron was among those quick to welcome the royal announcement.

“Many congratulations to the Duke and Duchess of Cambridge,” he tweeted. “I’m delighted by the happy news that they’re expecting another baby.”

Read: Prince George turns 1: Five things to know

Read: Royal baby name: The history behind George Alexander Louis

CNN’s Carol Jordan and Max Foster contributed to this report.