“People showed that a fetus could learn, was aware of elements of language, and preferred its mother’s own voice,” says Reid. But while such studies looked at hearing, touch, taste, and balance, vision was bizarrely neglected. A lot of researchers have looked at how newborns see the world, but most suspected that there was no way of doing similar tests before babies were actually born.

Reid proved otherwise. He and his colleagues essentially replicated studies that have been done with infants since the 1960s, showing that they prefer to look at human faces over all other kinds of images. The preference is so strong that even a vaguely face-like image will draw their attention—like a triangle of dots with two on the top and one at the bottom. If you’re being generous, you could argue that this pattern resembles two eyes and a mouth. But more importantly, it has many of the features found in actual faces—it’s top-heavy, symmetrical, and high in contrast. The resemblance is similar enough that babies are drawn to this pattern more than an inverted one, with two dots on the bottom and one on the top. And so, according to Reid, are fetuses.

First, he and his colleagues created a mathematical model that would predict how light would pass through a mother’s tissues, and what different images would look like to a fetus. Next, they used their model to calibrate two images—an upright triangle of red dots, and an inverted one. They shone these patterns into the bellies of 39 pregnant women, and then slowly moved the lights to the side. And using ultrasound, they could see that the fetuses would turn their heads.

Two images showing a fetus turning its head to follow three red dots. Kirsty Dunn and Vincent Reid

They didn’t always do so, though. They were more than twice as likely to track the movement of the upright face-like triangle than the inverted one—exactly the same pattern you find in newborn babies. “This tells us that the fetus isn’t a passive processor of environmental information,” says Reid. “It’s an active responder.”

It also confirms that the preference for faces isn’t the result of experiences that happen after birth. Some scientists have suggested that babies imprint on the first things they see—usually their mother’s face—in the same way that baby chicks or ducklings do. It’s very hard to test that idea: If imprinting happens and is important, it would be unethical to deprive a baby of that stimulus. “But this study rules that out,” says Reid. The preference already exists in utero.

Between 20 and 24 weeks into gestation, a fetus is upright in the womb, and its eyelids unfuse. It can then see, and what it sees depends on how light is bent, distorted, or blocked by the mothers’ body on its way into the uterus. Perhaps, Reid suggests, those patterns of light could influence the development of the fetus’s eye and brain, making the upper half of visual field more sensitive. That would create a bias towards top-heavy, face-like shapes.