A single-celled zygote (on day one of fetal development) is the precursor of a baby and the precursor of the placenta that will support fetal development. Thus, if we say the single-celled zygote is identical to the much later baby, we'd have equal reason to say the zygote is identical to the later placenta. But then, because of the transitivity of identity, we'd have to say baby and placenta are identical. So each of us was once a placenta! But no--that's surely absurd. So it cannot be true after all that the single-celled zygote is identical to the later baby (or placenta). The earliest that an individual (like you or me) comes into existence is when sufficient differentiation has taken place so that an entity exists that's the precursor of a baby and not the placenta. That time is at least several days after conception--when a blastocyst has become differentiated from the rest of the cellular material.
The logic here certainly makes sense. If a certain block of wood is a precursor of both a toy duck and a toy dog, it's not true that the duck is the same entity as the block of wood. You can't assert that identity, since there's equal reason to say the dog is the block of wood, and then you're faced with the absurd conclusion that the duck is the dog.
If it worked, the placenta argument would have some very important real-world implications, like forcing Catholic hospitals to change their policies on the morning-after pill for rape victims. They would have to admit that, for the first few days of development, the zygote is an entity in the same category as an egg or a sperm--a precursor of a baby but not the same entity as any future baby. Catholics will want to resists mightily, of course. They might say ... what?
What you might say is that a placenta is just a part of a fetus--essentially an organ, but one that happens to be (a) external and (b) temporary. A zygote is a precursor of a whole baby, but (of course!) also the precursor of the baby's parts. If we said the zygote is identical to a certain baby, we certainly wouldn't have equal reason to say the zygote is identical to the baby's liver, or heart...or whatever. The part-whole relationship between individual organs and whole baby blocks those identities. Likewise, arguably, for the placenta. It's just another organ, you might say, so a part (though external and temporary).
If we gave that response, would we just be making things up? Does it really make sense to think of the placenta as a part of a fetus? Is it more aptly thought of as a part of the mother? What is a placenta, really? It's curious that such an arcane biological-metaphysical question could have any bearing at all on what hospitals should permit rape victims to do. Surely there are many far more important ethical considerations. I certainly think so, so I'm talking about this issue merely "ad hominem" (in the technical sense, not the "street" sense), i.e. as relevant just to those who do oppose abortion on grounds that a baby starts to exist at conception.
If a placenta is a separate entity from a fetus, and not a fetal part, then thoughts about zygotes, placentas, and identity could show, quite decisively, that the very earliest conceivable start to any individual human being's existence is a little bit later--at least a few days after conception.