(Image Credit: PLOS ONE)

Nine months is a long time to hang around in the dark. But a new study suggests that fetuses yawn not out of boredom, but because growing is hard work.

British researchers used ultrasound scans to catch fetal yawns, a behavior that might be linked to early brain development.

"Fetus yawning is not contagious, nor do they yawn because they are sleepy," study author Nadja Reissland from the University of Durham said in a statement. "Instead, frequency of yawning in the womb might be related to activity-dependent brain maturation early in gestation."

Click here to see a face developing in the womb.

The study, published Wednesday in the journal PLOS ONE, suggests that more than half of all fetal mouth openings are yawns at 24 weeks of gestation. The frequency of yawning wanes after 28 weeks, according to the study.

Yawning isn't the only grown-up behavior seen in the womb: Fetuses also stretch and hiccup.