Question 5
Why can a foetus have a different blood group from its mother?
The right answer is the one marked above.
No exchange of blood takes place across the umbilical cord. The two circulations run close enough for substances to diffuse between them and no closer.
The cord does carry blood — an artery and a vein — but it is the foetus's own, which is what “the umbilical cord carries no blood at all” misses. The same fact explains a darker one: anything small enough to diffuse gets across, including disease-causing agents and some viruses.