Itachi's plan in life has been "Let Sasuke kill me, give Mangekyou so he can kill Madara" since the massacre. It's gained frills with time -- "drive Orochimaru out of Sasuke", for example, or "scare Danzou away from Sasuke" -- but apart from that, it stays the same; he's just walking along waiting for the time to implement it.
The missions he sabotages as often as he can get away with -- not often enough. Kisame watches him all the time.
Kisame is the one he likes most in Akatsuki, or dislikes the least, something in between. A competent, respectful partner who doesn't push with constant rivalry and doesn't cower or act paranoid. Sometimes he even manages to amuse him. Itachi sleeps with him mostly because why not. But it doesn't change the plan.
"Ah... Itachi-san? Did I ever tell you how my bloodline limit is different from baseline human in more than the obvious?" changes the plan.
"No matter. It's not like I'm going to keep it," he adds, and Itachi doesn't say anything because he doesn't know what to say. His family is dead, there's only Sasuke he can still claim as such : this has been true for so long that Kisame's "now there's more... now there isn't!" doesn't break through fast enough.
Madara noticing changes the plan. Because suddenly he's amused. Suddenly he's curious. Suddenly Kisame has no more choice, no more anything really.
Itachi isn't sure he feels very sorry for Kisame, dead in mind if not in body, who wasn't a nice man by any stretch of the word, but was still a competent partner the likes he rarely knew. But Itachi has seen the wall of eyes, dozens of little drawers with dozens of floating sharingans.
There's still a couple weeks before term when Itachi changes the plan. Kisame's body bleeds and bleeds when he cuts the child out. That's the only escape Itachi can provide him, who is much too big to be hidden in Itachi's shirt, under his large cloak. He lets Kisame to bleed himself out as he leaves the same way he came in, and large smears of red dry on his and the newborn's skin.
It's hours before he can clean and feed him. He swaddles him with the inside lining of his Akatsuki coat.
The baby's skin is a strange pinkish gray. But his eyes are normal human black, and he cries just like Sasuke used to, so long ago.
It's the first time in years Itachi doesn't have any idea what he's going to do. He just knows he's going to have to disappoint Sasuke.