It was the eighth month of pregnancy when Kyoya could barely walk that he started nesting. He had the strange urge to clean everything in the apartment over and over until it was spotless. And that's exactly what he did. He spent hours cleaning and scrubbing, jobs for the servants, he knew, but something inside of him said that they wouldn't do a good job of it, despite being specially picked for their jobs.
Tamaki had noticed it a few weeks into the behavior, noticing how the few servants, Tamaki couldn't stand too many people encroaching on his territory, spent less time cleaning, but the house still looked as spotless, if not more. Tamaki eventually narrowed it down to his darling Kyoya, who would only do a servant's job for an exorbitant amount of money.
Kyoya noticed also, to his horror, that in his last month, now almost fully confined to the bed, that he had begun stockpiling fabric and making a nest out of it, as in a bird's nest, for him and his child. Kyoya didn't understand his behavior, but he also started talking to his unborn child much more and keeping the servants out of his and Tamaki's room as much as possible. He reasoned that it was the protecting instincts of an expecting omega to keep their children safe and secure.
When The baby finally arrived, Kyoya was relieved and hopeful that his strange behaviors would soon disappear and he would forget about them, but as he and his alpha snuggled into Kyoya's nest with their child between them, he thought that maybe the behaviors weren't so bad.