It happened so slowly, through countless sessions of breastfeeding, battles over keeping food on the table and nights of sleep—713 nights of sleep, to be exact.

I recognized my firstborn's growth each time I packed away an outgrown size of clothes or looked back on his baby pictures in awe. But ounce by ounce and inch by inch, I didn't really comprehend how much it was all adding up. There was never a day where he simply seemed taller than the one before; never a night when it felt harder to lift his sleepy body into his crib.

Then it seemed to happen all at once: My first baby was no longer a baby and there was no denying it. That title now belonged to my newborn, who was positively tiny next to my walking, talking, gigantic toddler.

When I said good-bye just the day before leaving for the hospital, his untamed blond locks still felt like baby hair. I believed his full cheeks and doe eyes made him look like a baby. I thought nothing of the way I could pick him up and fold him into a hug just like a baby.

Then he came striding into the hospital room with his big boy cowboy boots and cautiously climbed onto the bed to meet his newborn sister—and I almost couldn't believe the transformation.

Call it hormones or whatever else, but it seemed to me that two births had occurred: That of my new baby and that of my new child.

This became abundantly clear just minutes later when "he feels hot" turned into a kind nurse taking my toddler's temperature and revealing he had a 103-degree fever. As my husband rushed my toddler home, I just sat there with my newborn in my arms and sobbed.

I cried with fear my new baby was infected. I cried with exhaustion over how much had happened in the last day. And, honestly most of all, I cried with overwhelming sadness that I couldn't be there for my toddler.

He was now, officially the "big boy." It wasn't just the stark contrast in their sizes—her, my 7 lb., 4 oz. newborn, and him, my toddler—but also the contrast in who they were expected to be. Whether he was prepared for it or not, he was no longer the baby of the house.

And yet, he was. At not-yet 2 years old, there were hourly reminders of his ongoing transition from a baby to a child. One moment, he could be asserting his independence and the next he would need a kiss for his boo-boo. Asking him for much more growth than he was already undergoing felt unfair.

What I didn't account for was how much growth he was eager to offer. From the moment he met his sister (even in his feverish state, which thankfully passed quickly), he was so kind, so gentle and so earnest in his responsibilities as big brother. He may not have asked to grow up as quickly as he seemed to, but he sure embraced the task.

In the time since his sister's arrival, he's become a fierce policeman of anytime I use the word "baby" in reference to him. "I'm not a baby, I'm a big boy," he sternly reminds me.

Although I laugh and agree with him, these past few months have shown me this isn't true: Yes, he is a big boy. But he will also forever be my baby, too.

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