Aang had known before Katara had.
He could feel the energy around her shifting and swelling, slowly, but noticeably. He hadn't known exactly what that energy was at first.
And then she'd rolled over to face him, a small, sleepy smile playing on her face, skin glowing in the gentle morning sunlight, and it had all clicked into place for him. His hands had drifted straight to her stomach, fabric bunching under his touch as he'd stroked her flat belly.
Katara had looked down at his hands, confused, but a smile blossomed on her face when she'd met his eyes, and he'd wrapped her up in his arms, knowing he'd never been happier.
Bumi was born to them nine months later; a little ball of energy, and a downright rascal. He tore around the house like a tornado, getting into everything, leaving no corner left unexplored.
That the first child of a master waterbender and the Avatar was born a nonbender didn't matter to them. He was their little boy and the love of their lives, and if his uncle Sokka spoiled him a little extra rotten, then they didn't mind.
A few years later Kya came to them, her kind heart and gentle nature certainly not outweighed by her cheekiness. She was nearly two when they first realized that she was a waterbender like her mother.
It had been bath time; filthy from a day outside, and Aang had carried her into the washroom screaming and crying, as she had apparently decided that she didn't want a bath that night.
The following tantrum caused his robes to soak up the entire contents of the tub.
Aang had carried her out into the living room with the biggest smile on his face, holding Kya aloft his head in excitement, her tiny little bare bum wiggling in an effort to get out of his grip.
"You're dripping on the floor, sweetie," Katara had said from her spot on the cushions, with Bumi asleep across her lap as she mended a torn tunic.
She'd waved her hand at him, the rapidly spreading puddle at his feet dancing up through the air around him before cascading back down over his head with a wet splatter.
Kya had squealed at the shock of water and then-
it was all across the room, dripping from the walls and coating the furniture, soaking into Katara's clothes, her hair drenched and plastered across her cheeks.
Katara had sputtered out a mouthful of water, looking between the two of them, eyes wide with disbelief.
And then she'd started laughing, and he'd laughed with her, Kya still dangling about him with a disgruntled frown on her little face.
That day holds a special place in Aang's fondest memories.
Katara had fallen pregnant once more about a year after, and familiar now with her shifting energies, Aang was, again, the first to know.
But something was different this time. A feeling. A something that he was sure he hadn't felt in a long time, and couldn't quite put his finger on.
A little boy was born to them, and when Aang had held Tenzin in his arms for the first time, he had known that their little family was now complete.
Of course, time passes quickly, and children grow up right before your eyes.
Aang had taken Tenzin out into the yard to play while Katara had gone out to the market, taking the Bumi and Kya with her. Tenzin's chubby little three-year-old legs had carried him as swiftly as they could, chasing his father around and around on the grass, his giggles and laughter infectious, until they'd collapsed into a heap, Aang tickling his little belly and Tenzin squealing.
And then Tenzin had sneezed, sending chunks of grass and dirt spraying up into Aang's face, and sending the poor toddler stumbling backward, landing hard on his butt, little eyes wide with shock.
Something primal had clenched in Aang's chest, an energy so close to his heart, something that he hadn't felt in a long, long time. He'd felt it before when Katara had become pregnant, and again when Tenzin had been born, but he hadn't quite understood at the time.
But now he knew.
The first to be born into the world in nearly one hundred and fifty years.
Katara finds him there sometime later, still covered in dirt and openly sobbing, a very confused Tenzin held tightly in his arms.
He finds he can't speak when she kneels down next to him, asking what's wrong. He just cries harder, tucking Tenzin under his chin and burying his nose into the little brown tuft of his son's hair.
Tenzin squirms in his arms, and Aang takes a deep breath. "He-," but Aang chokes on the words. He just holds Tenzin out to her, pressing a kiss onto his tiny forehead.
Somehow, Katara understands.
And then she's crying too, her arms around them both, and he's so, so grateful for the little family that they'd created together, and the love that they'd found in each other.
He had never really been alone, he's always been loved, and loved fiercely in return.
But something deep inside him alleviates as he holds his infant son's tiny hand in his own. A pressure that had been buried in him for so long that he'd almost forgotten it existed; lifting.
He's not the last.