Ron has never been very good at Arithmancy, much less basic arithmetic, and it isn't like he keeps a calender near the bed and marks down, carefully, every time he and his wife have sex, but he's pretty sure there's something wrong here. Nine months before three months ago was September, and he'd spent most of September in Bulgaria. He knows this because he sent a card home for Hermione's birthday, and she sent a letter back informing him of just how bad his Bulgarian was, considering how long he'd been there (almost three weeks, at that point).
Ron wanted to name the baby Arthur, or Harry, but Hermione thought it was bad luck. You name a baby after someone who's died, she said absently, and Ron tried not to wonder how long it had been since they'd heard from Harry. She suggested Perseus, and it wasn't until long after he agreed that Ron realized that was his brother's name.
Percy's not dead, he told her as they left the hospital a few days later. Hermione looked startled, then worried, before Ron explained. There was something funny in her eyes, something dark and sad, but when he gave her a hug and kissed the top of baby Percy's head it went away.
Ron doesn't know much about babies, either, and puts it down to that. It does have his hair, after all, and his eyes.