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.
"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."
Yes. But the mothers lungs would filter out any effects from the smoke before it is passed through the blood stream to the fetus.
The article is saying that her smoke 'directly effects' the fetus's lungs, which is not possible since the fetus does not breathe air until after it is born.
This study raises more questions than it purports to answer.
I think it has something to do with the liver transforming nicotine into a carcinogen and then transferring it to the baby.
It would have to be a chemical effect, perhaps from niicotine.