File photo: Kate, who is expecting her third child, is again suffering from hyperemesis gravidarum which plagued her during the early stages of her first two pregnancies. Picture: AP

The pregnant Duchess of Cambridge has spoken of the expectation placed on mothers to feel ‘super happy’ after they have given birth, saying: ‘One in four of us aren’t.’

Kate, 36, whose third child is due in April, was visiting a unit at Bethlem Royal Hospital in South-East London which houses mothers who have suffered severe mental illness in late pregnancy and early motherhood, ensuring they are kept with their babies as they receive treatment.

Briefed on research that found that, in the boroughs local to the hospital, one in four perinatal women experience some form of mental health issues, Kate said: ‘There’s an expectation you’re going to be super happy all the time, and one in four of us aren’t.’

‘These statistics are there, the research is there, but actually it’s getting the awareness out there for mothers to take it [help].’

Women spend an average eight to 12 weeks in the unit, before they are able to go home where they receive treatment for various kinds of mental illness including anxiety, depression and postpartum psychosis. It was not clear if Kate’s comments came from personal experience. Kensington Palace said she had been keen to learn more about the support offered to mothers.

Kate’s visit saw her wearing blue for the second day in a row – leaving onlookers wondering if it was a clue to the sex of her unborn child.

She chose a baby blue coat over a patterned dress, both by maternity label Seraphine.

Palace sources have always maintained that William and Kate made no attempt to find out the sex of their children before they were born.