Most of what we do know about the placenta comes from having studied it after a pregnancy is complete. This makes sense: A placenta is easiest to examine, from a practical standpoint and from an ethical one, once it’s available for scientific study. That happens once the mom and baby aren’t using it anymore.

“But in some ways, looking at that placenta after pregnancy is looking at a tree trunk stump and trying to understand what that tree was like as a sapling,” Spong says. “Just like the fetus grows, the placenta does too. And it’s very different at 20 weeks, versus 30 weeks, versus 40 weeks.”

Over the course of a pregnancy, the placenta grows bigger over time, and develops folds and crevices. It’s not just the structure that changes, but the function, too. In early pregnancy, placental cells are dedicated to invading the uterine lining—setting up shop, essentially. As the weeks go by, the placenta removes waste like the kidneys and liver; it provides oxygenation like the lungs; and it handles circulation. That is, if the placenta is working the way it ought to. Grave outcomes in a pregnancy are often believed to be related to problems with the placenta, but given the limitations of what’s known about the organ, it’s often impossible to determine what went wrong.

Even when researchers are able to study a younger placenta—say, when a woman gives birth prematurely—there are inherent limitations to what they can learn. Because preterm birth is not a normal outcome in a healthy pregnancy, a placenta from a woman who gives birth prematurely is not necessarily normal either.

“A lot of complications that a woman has—hypertension or even growth restriction in a baby—we think it might originate from a problem with the oxygenation [of the placenta],” said Afrouz Anderson, a post-doctoral fellow at the NICHD. “So there’s sort of a mystery we’re trying to solve there. The problem is that nobody knows what the standard placenta oxygenation [should be] to begin with, in a normal pregnancy.”

To find out, Anderson and a colleague have developed a handheld imaging device that can be placed on a pregnant woman’s abdomen. The portable device works by shining near-infrared light in the direction of the placenta, then measuring the wavelengths of reflected light to detect placental oxygenation. (Pregnant women are already accustomed to imaging technologies as a routine part of prenatal care; ultrasounds use high-frequency sound waves to produce pictures of a developing fetus.) Once researchers understand optimal oxygenation levels, such a device might be used to track placental oxygenation levels in women with high-risk pregnancies. One colleague at the NICHD compares this real-time tracking to a Fitbit—only one that you shine on your placenta, rather than wear on your wrist.