Itachi found out that the Kyuubi host had a child on the way early on, perhaps a week at most after it was conceived. An Akatsuki meeting.
"So the brat got laid. Do we give a shit?" Deidara asked.
Pain stared at him for a second and said, "not at the moment. In case we lose the Kyuubi host untimely, though, it might work as a back-up plan. Jinchuuriki descendents sometimes inherit similarly-sized chakra reserves, as some of us demonstrate."
Everyone looked at Kisame, who laughed quietly and inclined his head, as if to say, 'you got me,' and pretended not to see the flashing calculation in Kakuzu and Sasori's eyes -- 'ahh, so here's the first backup plan'. Itachi had of course figured that a long time ago. Kisame liked high stakes, so the reminder only prompted him to give a second chuckle. Deidara and Hidan totally missed it.
Itachi nodded slowly, and filed away the information. It would take the child years -- ten or fifteen -- to reach jinchuuriki levels, if it had the potential in the first place... which was very unlikely to happen, or no matter how hated the jinchuuriki were the villages would have set up breeding systems long ago. And even then, Itachi severely doubted it would be a match for the Kyuubi, the most powerful of the tailed beasts. Akatsuki's plans needed a Kyuubi, not a second Rokubi.
It was more likely Itachi would be long dead before any of this became relevant. The intelligence report went on, that tidbit soon forgotten.
+
Itachi found out that Pain's information had been wrong when he advanced on the girl -- Haruno Sakura, who would have been so happily ordinary if an unlucky fate hadn't thrown her in his brother's and Uzumaki Naruto's team, and from there on into Naruto-kun's bed.
Only then Sasuke threw himself on her and it wasn't merely the duty of protecting your teammates to the bitter end.
He'd seen that same look so many times, in similar eyes, that look of "I'm going to die but gods and ancestors, I beg you, do not let me die in vain." Terror and determination so deep, so mingled they became one single feeling. Do not let me die in vain, let something of me survive, let my life buy at least this.
(But he couldn't kill Sasuke just like he had killed all those people who'd ever looked at him that way. Sasuke had to live. Sasuke had to live.)
He only had one blood relative left in the world, Sasuke, the end of his line. It had to be that way.
Soon he would have two. Then what?
Later Kisame told him he'd been too cautious, bailing when he had, aborting their mission (the faint chance that the child might work as a backup sacrifice wasn't worth it, when it came with a pipeline that might siphon away all the chakra they needed to use.) He mocked him in that gentle, respectful tone, 'how about kicking him off and then stabbing, Itachi-san?' but Itachi didn't answer anything.
He had no answer to give yet.
+
Itachi found out that (Pain might not have been wrong after all) (Sasuke might have been bound by more chains than matrimony and fatherhood after all) he didn't know anything for sure anymore, on the night Sasuke and his bride exchanged their vows.
He was perched, in the dark, on the wall of the compound -- keeping watch with his back turned to the wall; one night of safety, my wedding gift to you, little brother -- when someone slipped out of the nuptial suite.
His brother. Leaving his bride alone.
Only to return, later on, towing along the Kyuubi child. Naruto. Towing him to the wall and up to the window and through to where his brand-new wife waited.
Oh.
He didn't move. He didn't leave, or turn around and come closer, or do anything but keep watch. That was his gift, and Sasuke didn't know he was giving it but that didn't mean Itachi got to take it back. His only gift. Because Sasuke must have known Itachi wouldn't hand him the life of the Jinchuuriki, or the life of the girl at the end of the pipeline, Sasuke surely didn't count on it, surely knew what would happen at the end of that road.
His little brother had always been foolish.
(He'd always loved Itachi first. Before.)
Itachi kept watch until dawn, and then he left.
(Nothing had truly changed, anyway. Nothing that mattered.)