Humans have a relatively hard time giving birth. Compared with other species, human newborns are large for their birth canal. This means that humans face the surprisingly high risk of our babies getting stuck during labor—which can be fatal for the mother, the newborn, or both. Some estimates suggest it occurs in as much as six percent of births worldwide.

The fact that this is an ongoing problem for our species has been a puzzle for evolutionary scientists. Why have evolutionary pressures not pushed the species away from such a common and fatal situation? According to an international research team including biologists, a philosopher, and a pediatrician, the answer lies in a complicated push and pull of several evolutionary factors between the baby and mother. But, the researchers find that there’s also a chance for us humans to tip our own evolutionary scales—with C-sections. Using a mathematical model, published in the Proceedings of the National Academy of Sciences, the researchers find that, in time, modern medicine could nudge human evolution off course, delivering bigger babies.

Falling off a cliff

To begin with, the sticking point is that there are competing evolutionary pressures for both the mother and the newborn, according to the study authors led by biologist Philipp Mitteroecker at the University of Vienna, Austria. Humans have big brains, which is a great thing, and you’re more likely to survive in life if you have a big brain (and a high birth weight). However, you’re more likely to survive birth if you’re small enough to fit through the birth canal. This means that the evolutionary pressures operating over an entire population push in the direction of larger babies—until suddenly, large becomes too large and the survival rate plummets.