Oof, that's a Cost there, if it has to be 12 weeks along, meaning they couldn't just find somebody who didn't actually want a baby and pay them to participate in the experiment. Though maybe Cheliax has some wizards who only do the Alter Self thing after they detect, or change their minds, and advertising for one of those would turn somebody up... it doesn't really solve the main problem, though, because he can't be sure that what works at 12 weeks works at 1 hour.
"If you Alter Self immediately after sex, then shift back, would I be able to check inside you and verify that the semen had vanished? Maybe I'd be willing to try for the further trust in that case, that semen doesn't persist in a vagina or uterus that you don't have... actually no, shit, I can't rely on that, because you could also produce that result with Prestidigitation even if it wouldn't ordinarily happen from Alter Self. No wait, counter-2-arguendo, if you can cast Alter Self and come back fast enough, I can have Detect Magic running the whole time, check your vaginal contents before and after, and verify that Alter Self was the only magic to affect you over that period..."
This is actually kind of fascinating, as Compounding Capabilities problems go. Maybe the eroLARP is deconstructing a computer game where the player needed to drag the spells from a list of available spells to form the correct structure to eliminate a pregnancy before they could fuck anybody, that sounds like how a computer-game version of an eroLARP would work.