Of the many stages and aspects of pregnancy, labor has always been one of the most mysterious. What starts it and how do contractions grow stronger during childbirth? Scientists at the University of Liverpool say they’ve figured it out.

It has long been known that the hormone oxytocin is important in stimulating contractions, which occur to help the baby move down the birth canal. What has remained a mystery, however, is how these contractions manage to increase and sustain their strength during the (sometimes) many hours of labor. After contractions begin, the uterine muscle tightens and squeezes blood vessels, which reduces oxygen and blood flow. This should, in theory, make contractions grow weaker, but as many mothers know, this is not the case.

"Laboratory tests have shown us that even when the hormone oxytocin is interrupted, surprisingly the muscle carries on contracting, and can grow stronger," researcher Susan Wray said in a press release.

This means that oxytocin, though it plays a significant role, is not the sole reason these vital muscle contractions continue during labor. Wray said this finding prompted her and her team to ask how this is possible.

Researchers started by looking for clues in studies on the heart, where an occurrence called hypoxic preconditioning causes cellular changes that protect it from more serious drops in oxygen. They found that the uterus reacts to the repetitive deficiency of blood flow and oxygen by triggering a previously unknown process in the muscle. The researchers are calling it hypoxia-induced force increase (HIFI).

They took this information to the laboratory, where they discovered that routine contractions in sample uterine tissue, when exposed to repeated dips in oxygen (hypoxia), responded by progressively increasing in intensity. Once HIFI was triggered, it could be sustained for many hours, consistent with what occurs in labor.

“This is an exciting discovery, not only for increasing our general understanding of how biological systems can respond to the stress of low oxygen but for opening up new research pathways into difficult labors,” Wray said.

This information could be key to understanding problems like prolonged labor, and the increasing number of births that result in emergency Caesarians, according to Wray.

Source: Wray S, Alotaibi M, Arrowsmith S. Hypoxia-induced force increase (HIFI) is a novel mechanism underlying the strengthening of labor contractions, produced by hypoxic stresses. Proceedings of the National Academy of Sciences. 2015