Prince William and Catherine, Duchess of Cambridge, are expecting their second child in April 2015.
The official statement seemed to suggest rumours the couple were expecting twins are false:
The duchess is suffering from acute morning sickness (or hyperemesis gravidarum), which also affected her during her pregnancy with their first son, Prince George. The condition affects less than 2% of pregnant women.
The condition often means sufferers require supplementary hydration, nutrients, and medication, the BBC reported.
The duchess has been forced to cancel a number of public events due to the illness, including her first overseas trip to Malta in September, but she will join her husband in welcoming the president of Singapore to London on Tuesday, the palace said.
The pregnancy and birth of Prince George set off a frenzy among media and royal watchers.