According to the statement
Catherine is in King Edward VII Hospital in Central London with Hyperemesis Gravidarum
. Acute morning sickness is fairly common during the first trimester of pregnancy. The Princess will remain in hospital for several days and then have a period of rest.
When the royal infant is born it will stand in line behind Prince William for the throne. In the past only sons were first in line for the succession of the throne but that changed in 2011 in the UK.
Princess Catherine's medical condition prompted the announcement. She is less than 12 weeks into the pregnancy.