[Moonman62]

I had trouble with that one myself. I don't have any explanation other than the mother's inhalation being taken in through the lungs to filtrate somehow from their to the fetus' circulatory system, which they share until the umbilicus is severed.

It would have to be a chemical effect, perhaps from niicotine.

[Froufrou]

Blood-brain barrier?

[aft_lizard]

http://www.pnas.org/cgi/content/full/97/23/12493

2'-Hydroxylation of nicotine by cytochrome P450 2A6 and human liver microsomes: Formation of a lung carcinogen precursor



Thats what would make smoking while pregnant dangerous to a fetus.

[CSM]

"It would have to be a chemical effect, perhaps from niicotine."

Better not eat tomatoes while pregnent then.