A new study from the UK reveals for the first time that babies in utero pack a powerful punch – more than 45 newtons, or ten pounds, of force.

A team of researchers observes that unborn babies kick in order to stimulate their skeletons – activity that is known to be critical for prenatal musculoskeletal development. Their pioneer study, however, measures unborn babies’ movements during the second half of gestation.

As the researchers note in the study published at the Journal of the Royal Society Interface, they used cine-magnetic resonance imaging (MRI) technology to track the babies’ movements and quantify their kick and muscle forces. They then applied the measurements to three-dimensional models of the fetal skeleton.

The authors found that kick force from the unborn babies rises significantly from 20 to 30 weeks’ gestation, and then declines sharply after that time, likely because the babies have less room in the womb to move around as they grow larger.

“Fetal kick force increased significantly over time, from approximately 29 to 47 N between 20 and 30 weeks … before decreasing significantly to 17 N at 35 weeks,” the researchers write.

They add the stress and strain experienced by the babies in utero as they grow is essential since “changes in movement in late pregnancy have been linked to poor fetal outcomes and musculoskeletal malformations.”

“This research represents the first quantification of kick force and mechanical stress and strain due to fetal movements in the human skeleton in utero, thus advancing our understanding of the biomechanical environment of the uterus,” the researchers note. “Further, by revealing a potential link between fetal biomechanics and skeletal malformations, our work will stimulate future research in tissue engineering and mechanobiology.”