Outside of the US, Tylenol is known as paracetamol; one of the metabolic intermediates of which is extremely toxic and since paracetamol is metabolised in the liver, it is there that it accumulates and has its toxic effect. The toxic dose of paracetamol varies among individuals because of the polymorphic nature of the genes for the enzymes involved and because of differing rates of metabolism of this toxic intermediate.
That said, the fatal toxic dose for a given individual is fairly high compared to the 500-1000mg (4000mg max. daily) recommended dosage. It would be considerably lower for a developing foetus, however, I am unsure as to how much paracetamol would be able to cross the placental membrane into the foetus' bloodstream; or indeed if the foetus would be able to metabolise the paracetamol (and thus produce the toxic intermediate, NAPQI).
Pregnant mothers should know better than to drink more than a small glass of wine, and certainly not in conjunction with any painkiller. Paracetamol is the lesser of many possible evils.
I hope this clears matters up?