Historian Kathleen Crowther sees a connection between Ancient Greek philosophers studying embryos and modern day abortion restrictions.

One way that manifests is in "fetal heartbeat laws" that outlaw abortion as soon as cardiac activity can be detected. It is often before a woman has discovered she's pregnant.

In her new book, Policing Pregnant Bodies, published Tuesday, Crowther points out that the fact that an early embryo has a beating pulse has been recognized since antiquity. The real influence of these ancient ideas is in the importance attached to the heart as "the seat of the soul and thus the essence of the person," she writes.

A scholar of the history of reproductive medicine, Crowther examines ancient metaphors that are still being used, describes the process through which early physicians came to understand fetal development, and explores the pernicious notion that a pregnant woman is the primary threat to the health of her fetus.

NPR talked with Crowther, who is a professor in the department of the History of Science, Technology and Medicine at the University of Oklahoma, to understand how these ancient ideas undergird the modern American concept of pregnancy in surprising ways.

This interview has been edited for length and clarity.

Selena Simmons-Duffin: I wanted to ask you about the metaphor that you tease at the beginning of the book, the "bun in the oven." Can you explain that metaphor and your objections to it?

Kathleen Crowther: It's a kind of cutesy little way of saying that someone is pregnant to say they have "a bun in the oven." That metaphor is really old – it first appears in texts by Hippocrates about 2,000 years ago to describe the process of gestation.

But if you think about that, if you've baked bread, the real work of baking bread goes on before you put it in the oven – the proofing the yeast and kneading the dough. That work takes time, it takes skill, it takes effort. Once you put the dough in the oven, all you're doing is waiting.

So why do we use that metaphor to describe pregnancy? That suggests that the active work has been done, presumably by the man, and then the uterus is just like this incubator that's growing this thing that was already made. I don't think most people who use that metaphor are being misogynistic. But I think it actually does come from a deeply misogynistic tradition of thinking about women's bodies as passive.

There are other metaphors from history that might be worth giving more thought to. One is – in the 17th century in England, a lot of texts on reproduction use the metaphor of the fetus being "a guest in the house." That's very much rooted to contemporary ideas about housewives and their duties of hospitality and care. But it suggests very different things about pregnancy, because taking care of a guest is work – it's work that you do lovingly – but it takes time and effort and skill. It suggests pregnancy is an active process – the pregnant woman is actively doing or creating something in a way that the 'bun in the oven' metaphor does not. Your oven doesn't need emotional or financial support, but someone hosting a guest might.

SSD: The first part of the book is about the heart. You write about all of the ways in which the idea of the heart is more than just an organ that pumps blood. It comes up in metaphors and songs, like 'Heart and Soul' and having a heart-to-heart conversation. After reading this chapter, I noticed this everywhere. Can you explain where the idea of the heart as the seat of someone's emotional self comes from?

KC: This is a really ancient idea – it goes back to the Greeks. Aristotle, in his discussion of embryology, said that the heart was the first organ to develop and that the heart was the seat of the soul. For the ancient Greeks, the soul was the animating principle of the body – it was that which gave the body life and movement and vitality. So for Aristotle, the soul is housed in the heart and the heart is the primary organ of the body. He describes it as kind of like the king if the body is a political state. And that idea continues to dominate thinking about the origins of human life for the next 2,000 years.

Even for physicians and philosophers who would distinguish the brain as the seat of cognitive faculties, the heart is the seat of emotions. There's this long-running sense that the heart is the essence of who we are, our emotions are what make us really human.

And so when people argue that cardiac activity signifies that a fetus is human, they're drawing on this very old set of ideas.

SSD: They're not just saying 'if a heart is beating, then there is life,' they're saying 'a heart is beating, there is a person – the essence of a person.' I don't think I had ever thought about those two different distinct meanings of 'heart' being drawn together into the same thing.

KC: For me, it was important to unpack. Physicians certainly understand a distinction between a fully functioning heart and the electrical pulsing of cells that will subsequently become the heart in an early embryo. But that argument holds, as far as I can tell, no weight with the people arguing that a heartbeat signifies human life is present.

One reason that Aristotle thought the heart was the central organ is that if you crack open chicken eggs a few days after fertilization, the very first thing that you'll see is a tiny little pulse. That's not a fully formed heart, but it's this tiny little pulse. For Aristotle, that was the beginning of the heart. And that's still what's going on with heartbeat laws. It doesn't matter that it's not a fully formed heart, it's the beginning of a heart, and that's enough to say that a human life is present.

SSD: Aristotle and his writings show up a lot in your book, including with some very wild metaphors, like that the uterus is filled with a kind of milk and the sperm makes it into a cheese that takes human form. Can you explain?

KC: Well, Aristotle is incredibly committed to the idea that the female body plays this entirely passive role in reproduction and the male role is active. So he uses a set of metaphors to explain what's going on, and one of them is how you make cheese. If you just have milk, it doesn't do anything, but if you add rennet, it begins to curdle and it takes on the form of cheese. So he describes what happens in sexual intercourse as something similar – menstrual blood is in the uterus, and when the active principle from the male sperm gets into that, then it forms into a baby.

Another metaphor he uses is that it's like a carpenter who turns wood into a house – the wood is obviously totally passive, and the carpenter is the active principle. So he's really committed to the idea that the female partner – and this is in animals and humans – plays this purely passive role.

As a scholar, sometimes when you spend enough time reading these ideas and sort of get into the mindset of the ancient Greeks, you forget how strange it is. And this is where it's been really helpful to teach this material, because students will be like, 'Whoa, wait a minute.' I did actually have this epiphany teaching that – that's really an incredible amount of mental gymnastics to go through, to say that the active role in procreation is like a few seconds of ejaculation and not nine months of gestation.

SSD: You also talk about how religious ideas play a role in concepts about the fetus and pregnant women. How does religious history on these issues fit into current political conversations?

KC: I can think of two important ways that this works. One is in those discussions of the heart. Christians adopted a lot of that language about the heart. It occurs in both the Old and New Testaments, but particularly in the New Testament – the heart is repeatedly referred to as the site of personhood and a site of interiority.

So that connects the idea of the heart to the more Christian concept of the soul. For many of the Greeks, the soul is the sort of animating principle, the soul is not always immortal. And so the Christian understanding of the soul as immortal and as having an existence separate from the body gets grafted onto these ideas about the heart and the relationship between heart and soul.

The other one is the notion that sex is purely for reproduction, so interrupting that in any way through birth control or abortion is a sin. That's really a new attitude toward sexuality and linking of sex with original sin that really starts with Saint Augustine.

I definitely, particularly in Oklahoma, have seen that play out in debates about abortion laws. Politicians supporting these will start talking about sexual morality and how the point of sex is reproduction. That's not a scientific idea.

For the ancient Greeks, but also in the Islamic world, sex is seen as something that's actually natural and healthy and something that most people need to do regularly to stay healthy. That doesn't mean there aren't strictures about who can have sex with whom – I don't mean it's some kind of liberal paradise. It's just that it's not really seen as exclusively for reproduction – that's a uniquely Christian view of it.

SSD: In the Dobbs Supreme Court decision last spring, Justice Samuel Alito famously wrote that "a right to abortion is not deeply rooted in the Nation's history and traditions." You note that a lot has been written challenging that. Can you just lay out some of what early Americans experienced with pregnancy and family-building to give some context?

KC: In the colonial period, in the early republic, fertility levels were generally quite high. People had fairly large numbers of pregnancies and childbirths. That's not to say that people weren't interested in controlling fertility, and although in many cases they may have been more interested in promoting fertility.

What we know is there were a large number of herbal remedies – methods that couples turned to for both birth control and abortion. And while none of these are, as a fact, nearly as effective as birth control or abortifacient drugs today, they do seem to have been used, and there doesn't seem to have been – in a lot of cases – any particular concern about these.

In the 19th century, it was perfectly possible for people to buy drugs known to have abortifacient properties like pennyroyal, for example. You could walk into a pharmacy and buy any number of drugs – that were not usually labeled abortifacients because that was illegal, but labeled "menstrual regulators" or "for female maladies" or things like this. That was really common.

SSD: Anti-abortion rights groups talk a lot about the Hippocratic Oath. The group that is challenging the abortion pill mifepristone in the courts right now is called the Alliance for Hippocratic Medicine, which is a reference to the fact that abortion is mentioned in the oath. So you paint a more complicated picture of that history. Can you explain a little bit about abortion and Hippocrates?

KC: One important thing is that the Hippocratic Oath only ever applied to a very small group of physicians – this was never universal in the Greek world. But yes, it does contain a line saying, I will not give a woman a "pessary" to cause an abortion. A pessary is actually something that's inserted into the vagina – a much stronger drug than those taken orally or applied externally, and thus much more dangerous.

It's significant, perhaps, that the oath forbids this one type of abortion, but it doesn't say, 'I won't give a woman abortion drugs,' it says very specifically, 'I won't give a woman a pessary.' So it could be that what the oath is forbidding is giving this very dangerous form of abortifacient drugs, because the oath also has prohibitions on giving poison.

The other thing, though, is that the Hippocratic Oath is just one of many Hippocratic texts. At one time, they were all attributed to the physician Hippocrates and scholars now know that they were written by a variety of physicians. But other Hippocratic texts describe abortions, including one of the most famous Hippocratic texts, On The Nature of the Child, which describes a physician inducing an abortion in an enslaved woman at the behest of her mistress, for whom the slave will lose value if she's pregnant. So it describes a physician telling you how to induce an abortion. And then other Hippocratic texts give drugs that will induce abortion.

So it's quite clear that abortion was going on in the ancient Greek world and that physicians were involved in it. To the extent that there are strictures against abortion in the ancient world, they tend often to be based on a sense that the fetus is really the property of the father and that abortions are wrong because they deprive the father of offspring.

Other physicians and philosophers endorse abortion in specific circumstances, as a way of population control for example, and certainly many physicians endorse abortion if carrying a pregnancy would be harmful to the woman. So the attitude toward abortion in the ancient world, even among the Hippocratic physicians, was a lot more complicated than I think those pro-life arguments make out.

SSD: You make the point that both those who support and oppose abortion rights point to history to support their arguments. So a big picture question is – what do you think the value is in examining these historical texts? It seems like there's a choose-your-own-adventure aspect to finding a historical text that's going to make your argument look rooted in the past.

KC: In the case of abortion, there is this explicit drawing on history on both sides. That's the history here that's more visible, whereas I think some of the other ideas that I talk about in the book are more submerged.

So one of the things that I thought was important to do was to actually connect these multiple histories of abortion. You could find people in the past thought all abortion was wrong. You could find people who said different things. You can find all of those precedents. But I really thought it was important to connect that history with this more submerged history of ideas about the heartbeat, with ideas of the pregnant woman being hostile to the fetus, for example.

It's not like all possible stories about abortion are right. I think the one that Justice Alito tells in the Dobbs decision – that there's no tradition of abortion in the United States – is wrong.