Even before I heard the bang on our front door, Christmas 2017 was special. Our son, Percy, was five weeks old and, since he was my parents’ first grandchild, there was a huge amount of excitement. My mum couldn’t wait for us to celebrate together at her house, a 30-minute drive from ours in St Louis, Missouri. She expected us there at 11am sharp. I was on the sofa feeding Percy at about 9.30 in the morning when I heard someone running up our icy front steps. After some frantic banging on the front door, I heard more footsteps, now running away. Assuming it was some last-minute gift delivery, I called to my husband, Rob, to open the door. “Laura,” he said, stepping back into the living room a few seconds later, with a stunned expression. “There’s a kid out here.” “Like a child playing a joke?” I asked. “No,” he replied. “It’s a baby.” Rob shouted that it was so cold, he was bringing it in. After I put Percy down in his basket, I turned and saw Rob holding a car seat. Sitting there was a baby in a blue jacket, staring at me. I was in a kind of shocked trance. This baby looked older than Percy; I guessed somewhere between nine and 18 months. He also looked healthy and well cared for. The second Rob put down the car seat and stepped out to knock on our neighbours’ door, the baby started to cry. My instincts kicked in. Unfastening the straps, I picked him up, held him close and began speaking. “There’s someone out there who loves you,” I found myself saying. “I’m sure of it. But if there isn’t, then I will love you. It will be OK, I promise.” It was as if he understood, because he stopped crying. How could anyone have abandoned a child, especially when it was -1C outside? Rob returned, explained that the neighbours weren’t in, and called 911. As I heard Rob say, “Er, we’ve just found a baby”, I held the boy close and felt a surge of anger. How could anyone have abandoned a child, especially when it was -1C outside? Waiting for the police was surreal. My mind was all over the place. I was sure there was a mother out there looking for him, as he’d responded to the kindness in my voice. But some part of me was also preparing for the alternative. I even thought about fostering classes I might have to take. When the police arrived about 10 minutes later, I was holding the baby and Rob had Percy, who was now awake. Turning to a woman who had emerged from the police car, one officer asked: “Ma’am, is this your baby?” I have never seen anyone move so fast. She flew up our steps in a split second, took him from my arms and seemed to collapse over him, sobbing. Knowing that he was now back where he belonged, I felt myself relax. As the two of them were led back to the car, Rob and I went inside to give our statements. Then we heard what had really happened. The mum and baby lived just a few streets away. After putting her son in the car that morning, she’d realised that she hadn’t locked her front door. In the second she stepped away, a teenage boy had stolen the car. Discovering the baby in the back seat, he panicked, leaving him on our doorstep and driving away. I shivered. What if he’d left the baby at an empty house, or if we’d already left for my mum’s? I’m a novelist and wrote down the experience in a Facebook post that night, hoping it would help me process it. Instead, it made it seem even weirder. Finding a baby on your doorstep on Christmas Day? It just sounded ridiculous. But the post was read by the baby’s mum and the next day I received a heartfelt message from her. His grandma also wrote: “Thank you for keeping my grandson warm.” I never saw the baby again, but I did see his mum at the court hearing about the theft, and she told me he was well. As for the teenage boy, I knew it could have ended in tragedy, but I felt sorry for him. He was a victim, too, in a way. A year later, we moved to a new neighbourhood. Percy, now six, has grown up hearing that for 20 minutes on Christmas Day he had a big brother. My dad is a vicar, and said it was a Christmas miracle. I don’t know if some higher power meant for us to take care of the baby that day. But I’ll always be thankful we were there to open the door when the knock came. As told to Kate Graham.