The back problems started in November, didn't they?
I was just thinking, during one of his oncourt interviews with Jim Courier at the Australian Open, Roger actually dropped a hint but nobody picked up on it:
Jim Courier asked Roger what he did during his holiday and he said he "lay on the beach, ate, slept, watched movies, watched the sunset, did nothing.....and you can guess what else" when the crowd started whistling.
This is pretty risque for a guy who's notoriously private and rarely talks about his life with Mirka. If she's expecting in the summer then she would have been in her first or second month at this time.