I look across the living room at my son as he puts together his latest Lego creation. I watch his fingers deftly pry apart the pieces and put them together just as he wants them. I take note of his broadening shoulders and lengthening neck, then notice that his face is changing too — his chin more pointed, cheeks less round, nose more distinct than when I last took stock.

As I stare at my youngest — my baby boy — the realization hits me all at once. Oh my god, I think, keenly aware of my heartbeat. He’s not little anymore.

I immediately scramble to figure out when the change occurred, to pinpoint the exact moment he left “little” behind. Looking at him now, it’s so clear that every trace of it is gone from his 8-year-old body. When did that happen? I can’t place it. It’s like I turned my back for a minute, and my baby was gone — just like that.

Now a big, smart, funny, amazing kid sits where my baby once did. He can read, ride a bike, make his own snacks, tie his shoes. It’s wonderful and sad, freeing and terrifying, all at the same time. I’ve loved watching my kids grow up, but each time, the “not little anymore” thing has hit me out of nowhere. It’s a bittersweet moment when you realize that you’ve completely left that major phase of childhood behind.

The little kid years are hard, but they’re also incomparably sweet. I adored having babies and toddlers and preschoolers. I loved all the newness of their world, and I loved watching them learn basic human skills like walking on two feet and talking in real words. I cherished their silky skin and soft, wispy hair. I relished the way they smelled, and the way their bodies would melt into me when they fell asleep. The little kid years are full of wonder and preciousness and magic.

There are major benefits to leaving that stage though. Now that all three of my kids are “big,” parenting is physically so much easier. I don’t miss changing diapers, washing the nooks and crannies of sippy cups, or wrangling jellyfish kids into car seats. I don’t miss toddler meltdowns (though tween meltdowns are for real). I don’t miss negotiating with tiny humans who don’t have an understanding of logic yet. I don’t miss having to watch over them constantly, lest they put something dangerous in their mouth or walk out the door and into traffic. With big kids comes freedom, and it’s glorious.

But everything is a trade-off. As my children get bigger to me, I get smaller to them. While I am relieved to not be their entire world anymore, I worry more about the impact the wider world will have on them. They are moving at light speed toward independence. They always were, of course, but now it’s painfully obvious. And letting them go is so much harder than I anticipated.

My oldest is 16 — almost an adult — but I swear she was just climbing into my lap with a board book in her hand. My middle kid is 12 and writes intricate stories for fun — wasn’t it just yesterday that she was learning how to scribble out her name in backwards letters? And now it’s my baby’s turn to go from little kid to big kid overnight, and my mother heart bursts with pride and heartbreak once again.

We all know it’s going to happen. Kids grow up. It’s what they do. It’s why we have them. But nothing can prepare you for it. Babies and toddlers seem like they’re going to be little forever, even though you see them changing before your eyes every day. They get bigger, but they’re still little and it goes on like that for years. And then one day, those little kid years are over — gone forever, without warning and without fanfare.

If you’re still in the little kid stage, hang in there. Some of it sucks, I know, and you won’t miss those parts when they’re gone. But the squeaky giggles, the baby-teeth grins, the chubby cheeks, the sausage toes — it all disappears. It’ll happen, and it’ll happen fast, probably while you’re not looking.

So drink it all in while you can. Smell their sweet heads, hold them as long as possible, snuggle them while they still want you to. Though they change gradually, one day you’ll turn around and won’t recognize your child.

Your little one will be suddenly be big. Before you know it. Just like that.

This post first appeared on Scary Mommy