Ever see Jurassic Park? First there's 'oooo-ing' and 'ahhhhing', then there's running and screaming. That's how it is with slash. First Severus' head in Harry's lap, first a hair brush (IN HIS HAIR), and soon it'll be a bed and the hair brush will be, um, better utilized.
Nah, I'm just teasing you. I think it's great you wrote mpreg, and it's a very nice moment you chose to portray so well. Good job!