And about exposure - yes, I think they have to absorb some particles during pregnancy. Other galactic civilizations are exposed to eezo technology for very long time yet rate of biotics isn't visibly higher then humans (except asari, of course). So whole process have to be more complicated and rare then simple "eezo radiations near pregnant woman".
Well according to Kaidan only 1 in 10 cases of a fetus being exposed to eezo does it result in biotics (something like 30% get sever health problems like tumors, 60% get no affect at all, and the last 10% are the only ones that develop biotics). So even if my idea is true, it mostly just means biotic mother's children have fatal health issues more than children from non-biotics and that even 60% of the children of such parents who don't have tumors end up with no biotics anyway. That's the thing, you think it would make biotics more common because you neglect to think about how little that would happen even with this idea.
EDIT, found a source:
This site has all Kaidan dialogue in ME1, and here's what he said. Got the numbers wrong though, it's 60% that have no changes and 30% that get tumors. Think I'll edit that part in the above paragraph.
http://masseffect.li...507.html?nojs=1
Neutral (Shepard) – What’s the survival rate? Seems like you beat the odds. How many didn’t make it?
Kaidan: Out of a hundred? Maybe sixty have no effect. Thirty suffer “adverse effects.” Little things like brain cancer. The other ten show enough ability to augment with implants. Not always permanent, though. Not like the cancer. Next thing you know, you’re out on Jump Zero. How’s a kid supposed to deal with that? A station at the edge of human space?