A US parenting influencer admitted to accidentally running over her young son with her car in a lengthy social media post — calling the incident a “true nightmare” for parents.

Kelly Hopton-Jones, a 36-year-old mother of two, wrote in an Instagram post that she was leaving to drive her daughter Lily, 4, on Wednesday to pick up doughnuts before her dance performance, while her husband and 23-month-old son Henry stayed home, The New York Post reported.

Ms Hopton-Jones – who shares parenting advice to over 63,000 followers under the Instagram handle “Hillside Farmhouse” – said that as she was leaving, Henry darted out of the garage and “in a matter of seconds” was run over by her car.

“I was driving,” the Texas-based blogger wrote.

“Our neighbours stepped in immediately and took Lily, and we rushed to the emergency room.”

Ms Hopton-Jones said X-rays of Henry’s chest, legs, and neck have all come back normal, and a neurological exam showed no signs of a head injury.

Henry, however, has fractures to his pelvis that will “take time to heal” and a few abrasions, she said.

Ms Hopton-Jones described the incident as “the worst day of our lives”, calling it “a true miracle” her son wasn’t more hurt.

“We keep replaying it, trying to understand how this could’ve happened when we were right there. I don’t have that answer,” she wrote, alongside pictures holding Henry’s hand while he lay in the hospital bed.

“It’s so easy to start questioning everything. If the morning had looked different. If B had gone to work like he normally would have, both kids would have been safely in their car seats.

“We could drive ourselves crazy with the what ifs, and honestly, we are a little bit.

“But accidents happen.”

This article originally appeared in The New York Post and has been reproduced with permission.