Kate Middleton Pregnant with Second Royal Baby
Duke and Duchess of Cambridge already have one-year-old son Prince George.

Kate Middleton may be forced to bow out of her forthcoming royal visit to Malta if her severe morning sickness continues to take its toll, a pregnancy expert has warned.

The Duchess of Cambridge is battling Hyperemesis Gravidarum, the same illness she was hospitalised with in the early months of her first pregnancy with Prince George in 2012.

After Clarence House announced that Prince William's wife is expecting their second child, she pulled out of the opening of Oxford University's new Chinese studies centre and was also unable to attend the opening ceremony and start of the Prince Harry's Invictus Games.

Caitlin Dean, from the charity Pregnancy Sickness Support and the author of Hyperemesis Gravidarum: A Definitive Guide, thinks it is unlikely she will be at the official commemoration of Malta's independence on Saturday.

"It's hard enough being in a car with this condition," Dean told People magazine. "A major part of managing the condition is making sure you are able to rest, and even if you can do short periods of being up and about, you have to rest afterwards.

"Feeling a bit better and then doing too much and relapsing is very common. Once you are feeling better, you want to get up and do things, particularly if you are a mother already, but you have to be careful not to relapse."