File photo: The Duchess of Cambridge is suffering from the condition for the third time. Picture: AP

London - Pregnant women with the severe morning sickness suffered by the Duchess of Cambridge can suffer mental health issues for up to six months after the birth, a study suggests.

Research presented at a conference will highlight the "emotional distress" of mothers-to-be with hyperemesis gravidarum (HG).

Those with it can vomit up to 30 times a day, leading to severe dehydration, malnutrition and hospitalisation. Kate is suffering from the condition for the third time.

In a study, published in Archives of Women’s Mental Health, the authors wrote: "Women with HG were more likely to report emotional distress compared to women without HG during pregnancy and six months postpartum.

"The results suggest that the increased risk of developing emotional distress may primarily be a consequence of HG."

Caitlin Dean, of the charity Pregnancy Sickness Support, said: "Research to be presented today demonstrates the mental health effect of this condition can be profound and doesn’t always end when the baby is born."

Clare Murphy, of the British Pregnancy Advisory Service, said women have for too long been expected to "put up" with their debilitating symptoms and should have access to the full range of treatments available.