As the first-time mother of a newborn, I am obsessed with time. I look at the clock over and over every day: checking the time at each feeding and starting anew the countdown to the next one, counting the hours my son slept last night, counting the minutes until I think he will wake up from his nap and determining whether I have time to take a shower.

I count these hours and minutes, but lately I'm finding myself counting days a lot, and, more fearfully, weeks. I count the days until his next doctor's appointment, the days I think I have left until I need to buy more diapers, the days until my husband's paternity leave ends and he has to go back to work.

The weeks I count with a clenched heart: the weeks since his birth. One slipped by almost without my notice, as labor and delivery complications meant we were hospitalized for the better part of those first hazy seven days. Then, just as deftly, week two dissolved like bath salts, bringing my son's extra 11 ounces, one inch and double chin. Now another half of a week has passed away, and some of his onesies are getting snug. He's eating more, making more faces. Changing.

These changes are why my heart physically aches and my throat feels tight when I think of counting weeks instead of days. It's unfair how quickly they go by. I've been given this miracle of a child, given this whole new love, but I know that every stage is as fleeting as a wisp of smoke.

The hospital stay that I had anticipated for nine months was over in the blink of an eye, and I lost count of the number of times I cried the day we were discharged. We were all getting stir-crazy in our tiny room in the high-risk unit, but this first milestone still wrecked me, and my face is tear-stained and puffy in all the photos.

Now the sticky, sand-colored residue from the adhesive that attached my IV to my arm and my C-section dressing to my abdomen has all but rubbed off, the bruises from the needle sticks have faded, and I only occasionally feel a twinge of discomfort as I recover from my surgery.

Every day, I strive to remember the tiny details of my son's birth, terrified that they, too, will fade. I fear the inevitable day that I look at him and he's no longer a newborn because then there will be the day he's not a baby anymore, then the day he's no longer a child.

The day will come when a yellow school bus takes him to kindergarten, when he slams his bedroom door, when he doesn't want me to kiss him anymore.

There will be a day when he doesn't live under my roof, when marriage and family will multiply the number of people in his life whom he loves more than he loves me.

All these things are normal and good, I tell myself, but I can't help but wince at what I've already discovered must be the essential grief of all parents: the measure of our success is how well we prepare our children to leave us. I find this to be humbling, empowering, and almost unspeakably cruel. I've been given a treasure, but I've also been given a ticking clock.

The only way I can ease my aching, too-full heart is to kiss his face as often as I can, stare back into his eyes when he looks up at me over his bottle, brush my cheek against his soft hair, run my lips over the wrinkles on his forehead. Most importantly, I have to believe that although he may forget it from time to time in the years to come, he will always return to the truth that I have been instilling in him from the moment I delivered him into this world: My son will only have one mother, and I will love him until the end of time.