hi

I know that it's easy to say "the unborn isn't a parasite because parasites and hosts are different species!" but there's something called intraspecific parasitism, where (for example) one bird will lay eggs in the nest of another bird of the same species.

Not a single scientific source refers to the zygote/embryo/fetus as a parasite, and that's for good reason.

Primarily (& there are other reasons) the relationship between the host and the parasite is that of conflict. The relationship between mother and child is not cooperative.

A typical host/parasite relationship is one where the host's immune system tries to get rid of the parasite, and the parasite either adapts or is killed by the host's immune system.

This happens when the host's immune system is functioning the way it should as well as the parasite's adaptability is functioning the way it should.
The inherent relationship here is conflict.

If the mother's body is attacking the fetus, we consider this to be a malfunction; a deviation from a correctly functioning organ system.

And the fetus's body does not adapt to attacks from the mother's body. Because the fetus does not function the way a parasite does.

And in many cases, we have discovered fetal cells inside of a previously pregnant mother, that were used to aid in healing.
So this means that while the mother is providing nutrients to the baby, the baby may actually help the mother by sending healing cells her way.

So in summary:

Some examples of parasitism within a species exists. It's called intraspecific parasitism.

A child is not a parasite because the mother/child relationship is inherently cooperative, whereas the host/parasite relationship is inherently contentious/combative.