Some differences between embryos and parasites.

[Esta publicación está disponible en español aquí.]

A few people asked us to respond to the “fetus is a parasite” TikTok. We posted some thoughts from Dr. Calum Miller here. I added to his points in this TikTok (transcript below video):

Yeah this is not accurate in terms of biology. There’s a lot in it that’s true but there’s a lot that’s left out which undermines the overall conclusion that the embryo is some kind of invading force tricking the woman’s body into accepting it.

The video is presented as the view of scientists but it’s just a pro-choice narrative laid on top of a biological facts.

It’s true the embryo has to have biological mechanisms in order to avoid an immune response because the embryo is a genetically distinct human organism from the mother. It is a different human being, as we all were at some point in our mothers’ uteruses. It doesn’t then follow that the embryo is a parasite. There are some similarities between the embryo and parasites like taking nutrients from a host and avoiding immune responses, but can you think of any differences?

For example, where does the embryo come from? Because if it’s a parasite it’s an endoparasite, which means that it lives inside the host’s body. But endoparasites don’t just appear in a host body. The malaria parasite is injected through mosquito bites. Roundworms you have to accidentally eat their eggs, which is super gross.

But we know embryos don’t enter from an external environment. They are created within our bodies, because our bodies are specifically trying to make that happen.

For example, we release an oocyte (which is the egg cell) on a monthly basis for decades. And on the membrane of the oocyte there is what’s called the ZP3 glycoprotein, and that serves as a sperm-binding receptor.

And once the sperm penetrates the oocyte, the oocyte gives off waves of Calcium ions which cause a cortical reaction which prevents polyspermy. Polyspermy is when you have more than one sperm penetrate the egg. This is especially interesting with the whole embryo-is-a-parasite narrative, because if you have polyspermy the resulting cell isn’t viable, so it would be a really terrible parasite because it’d be dead. And yet we have specific mechanisms in place to try to prevent that from happening, and to make sure that the resulting cell is viable, probably because we need to be able to make this happen in order to sustain the existence of our species.

That’s another major difference, by the way, between embryos and parasites. If the embryo-is-a-parasite narrative were true, and our bodies are trying really hard to fight off these invaders, then if we succeed our species dies out.

Even before an embryo exists, our bodies are preparing for the possibility of one. Our uterine lining thickens so that it would be ideal for binding a blastocyst. Estrogens thin cervical mucus so that it’s easier for sperm to penetrate. And that’s all when there is no other organisms involved, in case there might be.

Does any of this imply that we must have children? No. These are just biological facts. But they also don’t imply that the embryo is a parasite, much less some kind of aggressive invader.

You know what else is a biological fact? Abortion kills human organisms. I think if pro-choice people were okay with their position they would just accept that and move on. Instead they try to pathologize all of pregnancy, which is stunningly anti-female, by the way, in order to feel more comfortable with their political perspective. It’s just not a good look.

And as a quick follow up: please note parasites do not always have to be a different species as the host. Intraspecific parasitism is parasitic behavior between members of the same species. However the prominent example is usually brood parasitism, such as when a bird tricks another bird of the same species to take care of her eggs. Brood parasitism is a type of ectoparasitism (when the parasite exists outside the host body). I’m not aware of any examples of intraspecific (same species) endoparasitism (parasite inside the host’s body). And certainly all of mammalian reproduction isn’t just intraspecific endoparasitism. I say all that here: