If Severus' eyes were rather round when they were bantering about him smiling, at the latest announcement, he flat-out boggled: wide eyes, gaping mouth, the works.
"Pregnant?" he breathed, as a grin slowly dawned. "Lily, that's wonderful news!" He stood there, stiff as a board for a long moment as he looked searchingly at her; then with a huff he abruptly lunged at her, spidery arms folding around her in an awkward hug. "That'll be one extremely lucky baby, to have you for a mother."
He released her as soon as she started to speak about hexes. His expression shifted abruptly: a flash of apprehension, swiftly replaced by an intent, thoughtful frown. "It's a wise precaution; if you don't mind, I'd like to run some tests of my own devising. I'd need to make some preparations; what with the bedlam that the school will be once the students return, the earliest I could manage would be ... Saturday Possibly Sunday."