Yeah, I think basically people see the whole “[x] birth control method is 90% effective” or so on and get the wrong reading of it.
As it’s not really “roll a d100 and 90% of the time you roll over the ‘not getting pregnant’ DC”, it’s more like 10% of the time “the condom broke”, “your body didn’t handle the hormones correctly”, “you forgot to take the pills consistently” type deal, where the percentage is really “this is how many times people using this method got pregnant for any possible reason for failure”.
And for most of these, when they happen IRL and people don’t realize until they’re pregnant, it’s due to unawareness that certain symptoms or drug combos can = birth control failure, not luck. And with how attentive to detail Expat is in general, she’s more likely to have over-researched/planned precisely because of how much a trouble an unexpected kid would be for her.
So while there’s various failure points that could happen, there’s not many where Expat would be unaware they happened to the point of not getting to take a Plan B or similar equivalent. The only one I could really think of is when you get pregnant after a tubal ligation, and even that is more likely to produce an ectopic pregnancy than an actual pregnancy (aka the actual worst luck scenario for that one).
Phew.
And yeah, as for the whole “not getting powers” part of the equation, I feel Setback having powers while she doesn’t is a way bigger factor than Setback’s luck. I would expect there to be a higher chance of the kid getting powers even if Setback had just the serum powers with no luck component (not to mention the question of whether Expat herself has latent power genes of some sort).
And I admit I just don’t see how that takes away Expat’s desires or agency, because mothers don’t get priority or choice about genes in RL anyway unless they go the IVF route.
As for Teela, admittedly going off what details the related fandom wiki put in their summary about her, she’s more analogous to Wellspring’s meddling to me regards what could be viewed as problematic, not anything to do with Setback.