I must honestly say, I thought about Sakuraiba mpreg (not in fic, but in Sims XD) just a few days ago, and I decided on knocking up Sho rather than Aiba. Which has a little to do with the fact that I think one day Sho will be one of the best dads in the world, and a load to do with me thinking... Aiba is such an energetic and emotional person. If he was pregnant, he'd have to be careful all the time and couldn't do fun things anymore. Also, can you imagine what pregnancy hormones would do to the poor guy? He cries easily enough as it is, combine that with hormones and by the time the baby comes he and Sho (and probably the rest of Arashi) will be a nervous wreck.
But that's just my personal opinion and otherwise the fic was pretty cute ^^, when one ignores the whole "even the doctors don't know" concept (I normally only read mpreg in HP fandom, but again, personal taste and all that). Oh, and the baby will be ADORABLE no matter who's the "mommy", so... XD