Question: how long would it take to make a humans first red blood cell?

David Christensen answered on 21 Nov 2013:

That’s a tricky question and I don’t know enough about development, but I’ve had a quick look on wikipedia (!) and elsewhere with Google. It seems that the heart and first blood vessels develop in the fetus at about 4 weeks after fertilisation, so I guess you could say that it is about 4 weeks for the first red blood cells to form. It takes that long because there are other things being done in the fetus and because the fetus isn’t big enough to need a blood supply to deliver nutrients to its cells before this point. If we wanted to make a red blood cell in a lab, using stem cells, I guess it would be quite a lot faster.

I hope this answers your question.