Megan Kibby, 33, said she never expected to become a grandmother at such young age. But when Tassi, the goat she raised with her business partner, Jason Lewis, gave birth to twins in the midst of a goat yoga session in Suwanee, Georgia, on Sunday afternoon, she was delighted.

Ms Kibby and Mr Lewis run Red Wagon Goats, a goat yoga business based in Stone Mountain, Georgia (Ms Kibby teaches the yoga). They knew that Tassi was going to give birth soon. Another of their goats, Emmylou, had given birth to twins the previous day and, as Ms Kibby put it, “when one of them goes they all go.”

They did not think it would happen in the middle of a class.

Red Wagon Goats was holding a goat yoga fundraising event at one of city’s parks on Sunday, when Tassi — named for the Tassili n’Ajjer site in Algeria — began to paw the ground, looking for a place to nest. Mr Lewis picked her up intending to take her to a trailer where she could give birth more privately, when she began to deliver.

“She’s having this baby right here,” he said. (Several days later, the story of the birth was featured in The Atlanta Journal-Constitution.)

Mr Lewis and Ms Kibby are just two of the many goat owners who have joined the goat yoga craze. It started in 2016 and has spread from the Pacific Northwest to the rest of the country, and others, including Canada, South Africa and Australia.

Goat yoga did not spring from some Burning Man brainstorm. It was born at a children's birthday party in Albany, Oregon, at the home of Lainey Morse, who had between six and eight pet goats at the time. One of the parents in attendance, Heather Davis, was a yoga instructor. Watching the kids cavort, she had a eureka moment: "Why not combine goats and yoga?"

The concept took off, fuelled by goat-lovers on Instagram. News of the trend was spread by the news media. But most viral stories die early; goat yoga is stronger than ever.

“I’m sure that everyone in the world thought that this was going to be done in five minutes,” Ms Morse said. “But it’s not. It keeps growing.”

Ms Morse’s company, The Original Goat Yoga, now has 10 locations in different states and hundreds of competitors, (one of which, based in Arizona, snatched up GoatYoga.com). She has done goat yoga sessions with 10 of her 13 goats for employees of companies based in the area, including Intel, Adidas and Nike.

She says that “no one has ever gotten hurt or head-butted” in a session but that there are plenty of goat-related distractions.

“Sometimes they sneeze right in people's faces and the whole class just erupts in laughter,” she said. “I’ve had over 3,000 people through my class and only one person has gotten peed on, so that’s pretty good.” And “goats do poop. If people have a problem with that they probably shouldn’t come.”

But she’d never heard of one giving birth during a class.

Tassi was the first goat that Ms Kibby and Mr Lewis raised together. The first twin that she gave birth to on Sunday, Sue-Anna, has “bright blue eyes just like her momma,” Ms Kibby said proudly.

The second kid came as a surprise. Clouds had begun to roll in during Sue-Anna’s birth and as the goat’s owners tended to her and the class cooed over the baby, lightning flashed and the hooves and head of another kid began to emerge. They named him Storm.

“We have grandkids now,” Ms Kibby said.

Sue-Anna and Storm joined the other babies born over the weekend, Electra and Orion, bringing Ms Kibby and Mr Lewis’s head count up to 35. Red Wagon Goats holds family friendly yoga classes as well as sessions for adults 21 and up, after which they serve wine and chocolate. (Private events cost $40, or £34, a person for a minimum of 10 people.)

They are hired by a surprising number of bachelorette parties. At one such party recently, Ms Kibby said, “Half the girls were from the south and half the girls were from New York and the New Yorkers were screaming if the goats got near them.”

But most people who attend their sessions have a hard time resisting the animals. At goat yoga, for example, the instructors tell their students they’re free to abandon the yoga part of the class at any time.

“I am not delusional enough to think that they all are there to work out with me,” Ms Kibby said.