My 9-month-old could have died because my mother left him alone for "just a minute"

My husband, 3 yo son, and myself were outside trying to teach him how to ride his bike. It was ~90° so my mom insisted we kept the baby inside, she would watch him.First, I vacuumed the carpet then did a spot check for anything the baby can put in his mouth. I set down a big blanket, put up his wooden playpen I made for him, and put his toys in there with him.We're outside having a blast, trying to teach a 3 yo the concept of peddling is a lot harder than I thought. At one point I notice my mom outside without the baby.When I asked where the baby was, she said he's inside, playing. That he was "safe because she latched the playpen". Well, I still wasn't having it. I may be a young mom, but I understand babies are not to be left unattended.After picking him up I noticed he needed a diaper change. As I'm carrying him to his room, I'm tickling him, making funny faces, etc. and he's laughing. I lay him on the changing table and he looks at me, smiling. He starts coughing so I figured it's just spit up or spit went down the wrong pipe.After a few seconds, I don't know what possessed me to do it, I roll him onto his stomach, smack him three times (a little hard) in the middle of his back, until I can hear him cry. I pick him up, and a fucking penny had come out of his mouth.After I went to get him at no time did he have his hand anywhere near his mouth. We were laughing and playing and he had a penny in his mouth, unbeknownst to me, the entire time. I guess I knew something was wrong subconsciously when he was "coughing/choking". When no sound came out, my subconscious(?) probably thought back to the babysitting class I had taken 10 years ago. When he started coughing, the thought "Oh he's not making noise he must be choking" never entered my head, it was all a reaction I guess. And I'm very grateful it worked out the way it did.I honestly don't know how long my mom had left him alone for. Had things happened differently, we could've come inside to a blue-faced, dead baby.Tl;dr: If your baby is "coughing" but not making noise, they could be choking. My mom left my 9-month-old alone for however long. While alone he put a penny in his mouth. He didn't start choking on the penny until I was changing his diaper. I smacked the penny out. via /r/Parenting http://ift.tt/2tm4HIp