As any pregnant mama knows, babies do a whole lot of moving around in the womb—usually in the middle of the night when we’re trying to sleep. As the weeks go by, those jabs and kicks start feeling stronger.

It turns out, though, the purpose on baby’s end isn’t just to get in a workout session, but to actually send vital signals that will help his or her body continue to develop properly.

According to a study published this month in the journal Development, kicks and movement stimulate molecular interactions that prompt the cells and tissues of the embryo to build a “functionally robust yet malleable skeleton.”

This means that kick may be telling the body it needs to have a bone covered in cartilage at the joint, or a jab over there may send the signal to increase bone strength.

For the study, researchers from Trinity College Dublin were able to control the fetal movements of chick and mice embryos. This gave them some interesting insight on what movements result in signals to boost cartilage or bone production.

“Our new findings show that in the absence of embryonic movement the cells that should form articular cartilage receive incorrect molecular signals, where one type of signal is lost while another inappropriate signal is activated in its place,” says co-author Paula Murphy, a professor in zoology at Trinity College Dublin, in a press release. “In short, the cells receive the signal that says 'make bone' when they should receive the signal that says ‘make cartilage.’”

These findings have cool implications not only for pregnant mamas thinking about what their babies are doing all day, but also for researchers seeking to improve treatments for joint injuries or diseases.

In other words, the movements are making babies stronger and healthier for when they’re born—and may help them decades down the road when researchers are better able to treat osteoarthritis.

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