On ranking embryos best to worst

In the old days, IVF clinics just eyeballed an embryo and picked the one that looked best. Then along came testing. For a fee, an embryo could be biopsied to see if it had something wrong with it — the incorrect number of chromosomes, say, or a particular unwanted gene.

But what about all the embryos that make the cut? They can't all be equally good. How…

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