Why Do Babies Kick? It's a Workout for Strong Bones and Joints

They're not just saying hello.

It’s easy to confuse the phenomenon for a bad case of gas, but moms-to-be start feeling flutters and waves in their wombs about 18 to 25 weeks into pregnancy. These sensations are caused by baby kicks delivered with 10 pounds of force — a natural but mysterious anomaly that scientists finally explained in a study released last Monday.

Unborn babies participate in rough-and-tumble action because they need to develop strong bones and joints, researchers from Trinity College and the Indian Institute of Technology explain in the journal Development. Their analysis of chicken and mouse embryos showed that in-womb movement kicks off crucial molecular interactions: Embryonic cells use biological signals to direct them to the right areas of the growing body so that they can differentiate into various types of tissues. The movement of the baby, the researchers discovered, triggers those signals, which in turn leads cells and tissues to turn into strong and effective bones and joints.

Fetal legs move with 10 pounds of force per kick.

Wikimedia Commons

If an embryo doesn’t move and those vital signals are missing, a baby can end up being born with abnormal joints and brittle bones. The link between signaling and fetal movement, the researchers explain, allows the skeleton to become strong and malleable. Movement directs certain cells to become resilient bones and other cells to become the smooth lubricated cartilage that’s necessary at the joints.

“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,” explained co-author Paula Murphy, Ph.D., in a statement released Tuesday. “In short, the cells receive the signal that says ‘make bone’ when they should receive the signal that says ‘make cartilage.’”

Besides explaining the base-level need for a baby’s kick, this study also provides new insights into how healthy joints are made, which has previously eluded researchers. By having a better understanding of how embryos form articular cartilage, the researchers hope to discover ways to regenerate cartilage from stem cells — in turn, providing treatment for people with joint injury and disease.

“The relative lack of understanding around how cartilage was directed presented an unfortunate knowledge gap because there are many painful debilitating diseases that affect joints — like osteoarthritis,” explains Murphy. “And because we also often injure our joints, which leads to them losing this protective cartilage cover.”

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