The new royal baby will be born fourth in line to the throne. As a sibling to Prince George, the Duke and Duchess of Cambridge's second child will not be expected to be crowned sovereign.
But second-born royal children - often dubbed the 'spare to heir' - have on occasion ended up as monarch.
The country's last king, George VI, was not meant to accede to the throne and only did so when his older brother Edward VIII abdicated over his love for American divorcee Wallis Simpson in 1936.
Kate's first pregnancy was revealed when she was just a few weeks pregnant after she was admitted to hospital suffering from severe morning sickness in December 2012.
Hyperemesis gravidarum, which affects 3.5 per 1,000 pregnancies, causes severe vomiting and can lead to dehydration, weight loss and a build-up of toxins in the blood or urine, called ketosis.
After it was announced the Duchess of Cambridge is pregnant with her second child we look back at her last pregnancy spent in the spotlight.
Kate and William's second child will join second-born Royal children as being known as the "spare to heir".