Duchess of Cambridge poses with staff at Kingston Hospital’s Maternity Unit.

Kate Middleton, the wife of Prince William, wrote an open letter of thanks to Britain's midwives on Friday after spending several days observing a hospital maternity unit, calling their work humbling and crucial.

Kate, 37, the Duchess of Cambridge, the mother of George, 6, Charlotte, 4 and Louis 1, spent time last month at Kingston hospital in southwest London, accompanying midwives on their daily rounds and going into people's homes with them.

"I was truly touched by the trust that people placed in me, sharing their experiences and voicing their fears openly," she wrote.

Duchess of Cambridge talks with staff during a visit to Kingston Hospital's Maternity Unit in London.

She also visited clinics and post-natal wards.

"No matter the setting, I was continually struck by the compassion that those of you I spent time with showed and the incredible work ethic you demonstrated on behalf of your entire profession," she wrote.

"You are there for women at their most vulnerable; you witness strength, pain and unimaginable joy on a daily basis."

She said she had devoted a significant amount of her work to children's early years, the period between pregnancy and the age of 5.

Duchess of Cambridge, looks at a baby with its parents during a visit to Kingston Hospital's Maternity Unit in London.

Midwives' role in supporting this phase extends far beyond the complicated task of delivering a baby successfully, she wrote.

"The help and reassurance you provide for parents-to-be and parents of newborns is just as crucial," she added. "It goes a long way in building parents' confidence from the start, with lifelong impact on the future happiness of their children."

She noted the World Health Organisation has designated 2020 the "Year of the Nurse and Midwife" in honour of the 200th birth anniversary of Florence Nightingale.

"Next year the world ... recognises and celebrates the humbling work that you and your colleagues do day-in, day-out to improve the lives of others," she wrote.

World Health Organisation has designated 2020 the "Year of the Nurse and Midwife." 

"You are there for women at their most vulnerable; you witness strength, pain and unimaginable joy on a daily basis."

Reuters