Staying indoors for a month without bathing while following a strict diet is not something many British people would willingly do, yet millions of new mothers in China and across Asia practice postnatal confinement in the weeks after they give birth.

When Prince William's wife Kate Middleton, the Duchess of Cambridge, was pictured walking out of hospital just 10 hours after giving birth in 2013, Chinese mainstream and social media was flooded with comments by people who were shocked that she was up and about so soon.

"Foreigners are strong so they don't need postnatal confinement!" one person commented at the time.

Grace Wong, 37, a pharmacist living in West London, said: "Postnatal confinement is supposed to give you a solid foundation for future health. I don't know if it's true, but all my friends and family do it."

Grace, whose grandparents are from China, moved to the United Kingdom from Malaysia in 1998. When she had her first child in London, her mother flew out from Malaysia to perform the role of "confinement nanny".

Her mother stayed with her for 40 days, cooking meals and preparing herbal remedies to aid circulation and promote milk production.

Postnatal confinement is big business in China, where career "confinement nannies" can earn well above the average wage.

Grace is planning her second pregnancy, and will use a new service launched in London this year called Healthy Mummies, where traditional Chinese confinement meals will be delivered to her house for 28 days.

Macy Ng, a pharmacist who co-founded Healthy Mummies with accountant Jo Neoh, said: "There are lots of women in London from places with the tradition - China, Singapore, Malaysia. Eventually, our plan is to market it to everybody, but for now, our core business is Asian moms."

The new service focuses primarily on the dietary tradition. A day's menu may consist of pig trotters in vinegar and ginger, red date tea, and lemongrass chicken soup.

Ke Songxuan, a traditional Chinese medicine doctor who runs the Asante Academy for Chinese Medicine in London, said most aspects of postnatal confinement are cultural customs.

"I recommend rest, good food, and keeping warm," Ke said. "Dark vinegar is good for blood circulation. Chinese white mushrooms help with milk production. Not bathing or washing hair might come from a time when there wasn't good heating. I would encourage women to stay clean to prevent infection."

Fredric Willmott, a consultant obstetric gynecologist in London, said new mothers in the UK are generally discharged the day after giving birth.

"High-risk patients stay in hospital and are reviewed by doctors with an individualized plan for discharge. Even so, it is rare for people to stay more than three nights," he said. Wong said she will stick to the diet the second time around, but won't be rigid when it comes to not washing or leaving the house.

"The first time, I couldn't bear it, I washed my hair in the first week," she said. "And I did leave the house in the third week for some fresh air. My Chinese friends who do proper confinement told me not to do that. They weren't judging me, they said it in a caring way."