After all, there aren't that many Suvian babies about. If they all were tested to death, you'd rapidly run out of test subjects.

We do kill Sue children though. Have there been any actual Sue babies, as opposed to the children of Sues, and if so, what happened to them?

No, it hasn't been explicitly stated I don't think, just alluded to. Bend your brain if you want, but I think the rest of us just assume it works and leave it at that. Could be cool though. :P

Ron was, for various ridiculous and unexplained reasons, turned into a girl and transported to a weird alternate dimension which looks sort of like The Arabian Nights except everyone has Japanese names, ended up posing as a handmaiden to the Princess Sue, and was randomly raped by a guard (see, that's why guards in Real World harems were eunuchs). Doctor employed by the Princess heals "her" up and casts a spell to see if "she's" pregnant ... so actually it's a couple of hours, not a couple of days. Should sperm even have met egg yet? But then the story goes on for a couple more days, so it'll be implanted and such by the end.

(Then Ron's quite understandable fright is literally handwaved by the Princess saying "oh, women get emotional when they're pregnant!", thus making me want to go on a homicidal rampage, and the characters are used as mouthpieces for an anti-abortion rant. And it turns out that Draco doesn't know what abortion is - I really doubt the wizarding world would be totally ignorant of that concept ... hehe, "concept", excuse pun. See why I want this fic to die?)

Could be called a zygote, but you'd already have cell division after two days. It's an embryo up until the seventh or eighth week, at which point we call it a foetus.

I'd say spike the drink.