Medical imaging's come a long way in the past few decades—but it's now so good that doctors can use it to tell the difference between a baby opening its mouth and it yawning.

A team of researchers from Durham University, has been analysing data from 4D ultrasound scans which create three dimensional renderings of unborn children that depict motion in real time. Through studying the video, they're able to accurately say whether a baby is yawning or just merely opening its mouth. It turns out that younger unborn babies yawn the most, but it's not clear why. The research is published in PLoS One. Nadja Reissland, one of the researchers, explains:

"Unlike us, fetuses do not yawn contagiously, nor do they yawn because they are sleepy. Instead, the frequency of yawning in the womb may be linked to the maturing of the brain early in gestation."

The doctors speculate that the yawning might be a mechanism through which babies can develop brain functions for movement. Regardless of why it happens, though, it's damn cool that scientists can see. [PLoS One via Guardian]