Every firstborn I know has a story like this. Mine has me on a train at two weeks old, on my first official field trip. My parents interpreted my newborn foot twitching and eye blinking as signs they had a budding zoologist on their hands. It’s the only thing that can explain why they carefully assembled a baby bag on a sweltering August morning (I have no idea how they kept my formula from spoiling) and boarded the three of us on a train to Penn Station for the hour’s trip into Manhattan, followed by an hour-long ride on a crowded subway. Destination: Bronx Zoo.

From what I am told, it was all going swell until the elephants. My father held me up to face the animals. He sweetly told me what they were, (not the scientific classification, I hope) and added, “They are gray, Linda, gray.”

At that point, a man standing next to us with his older kids began to laugh. My father, experiencing the self-doubt that soon becomes a parent’s faithful sidekick, felt the need to justify himself: “She can see colors!” he said indignantly, and we went on our way to the lions.

Fast forward 11 years. I was now the big sister of the family. One afternoon, my younger brother burst through the front door with more than his usual excitement. His kindergarten class was going on a trip, and he bounced around the living room with all the details. My mother was standing at the kitchen sink. Surprised he was making so much noise, she walked into the room, drying her hands. She watched him jump from the couch to the chair, shouting, “We’re going to the zoo!”

After a few seconds, it sunk in.

“You’ve never been to the zoo?” she asked.

“Nope!” he said. Then he went in for the easy lay-up:  “And we’re going on a train!”

I heard my mother tell this story often. She had a knack for timing and making people laugh. But at the end of it, I always sensed there was a little sigh. Maybe a tiny regret that they had been so single-minded with their firstborn and wished she’d had a little more left for that third kid. Or at least hadn’t been so surprised to hear he’d never been to the zoo. Or on a train.

It happened to me, too.

My first son’s baby book could have won a Pulitzer. It is a perfect record – in excruciating minutiae – of his first two years. My second son’s baby book is half as long, even though he reached the same exact milestones. When my daughter came along, she had two brothers who were barely out of toddlerhood and a mother and father who, on their best days, were in survival mode. Her nearly blank baby book taunted me most – more than forgetting to pay the mortgage or letting the laundry pile up into a health hazard. I kept promising myself I’d catch up and fill it in.

When she was 12, she found it on my bookshelf and noticed how measly the contents were. She took it upon herself to fill in the missing lines with the most rudimentary facts.

“How old was I when I sat up?” she called from the next room.

“Five months,” I cheerfully guessed. I didn’t dare add, “give or take a month.” And now I tell that story with the same sigh I heard in my mother’s voice when she told hers.

Maybe, instead of feeling guilty that we didn’t keep up our laser-focus as we added babies to the family, it’s okay to say, “I loved you all differently.” After all, I wasn’t the same person on my initial ride on the parenting roller coaster as I was when my third baby landed in my arms. The first time I was white-knuckled, and every decision felt do or die. By the third, I rode with my hands high above my head but also aware of all the things I had to let go. It was different ride every time.

Though I could hope to be a good mother, I wasn’t the same mother. There was always enough joy for all of them – it was just different joy. And plenty to go around. True for that firstborn, who gazes cynically at the elephants through two-week-old eyes that can only see 12 inches in front of her face, and for that last one, who can’t believe his good luck when his chance comes to get to the zoo. On a train.