Royal baby: Kate's condition one of perils of pregnancy

Hyperemesis gravidarum is not a rare condition, according to midwife Rachel Cassie.

Charlotte Bronte probably died of it but these days hyperemesis gravidarum - the severe form of morning sickness experienced by the Duchess of Cambridge - is easily treatable.

Kensington Palace announced Kate's second pregnancy early because it is also marred by the condition that put her in hospital during her first pregnancy with Prince George.

Watch: Prince William: 'Thrilled' by second baby

Hyperemesis gravidarum, which translates as vomiting pregnant, is not a rare condition, according to midwife Rachel Cassie.

About 90 per cent of pregnant women would experience some nausea in the first three months but a smaller number, around 15 per cent, would have severe vomiting to the extent they needed intravenous fluids, antibiotics and medical management, Ms Cassie said.

Sufferers are usually diagnosed around the sixth week of pregnancy and can vomit up to 30 times a day, or retch after every meal and drink.

"Some people will draw the short straw and have a really miserable time of it, as opposed to feeling just a little bit seedy," Ms Cassie said.

English novelist and poet Charlotte Bronte was 38 when she died in 1855 during her first pregnancy, with many biographers suggesting her death was the result of dehydration and malnourishment due to vomiting caused by the affliction.

Ms Cassie, an independent midwife in Hamilton and former GP, said the majority of pregnant women would feel better after 12 to 14 weeks, or as the first trimester ended, but many continued to be sick until the baby was born.