The general course of child labor is well documented, but many mysteries—such as what starts labor in the first place—persist. Now scientists at the University of Liverpool say they've discovered what causes contractions to strengthen during labor, even though the tightening uterine muscle squeezes blood vessels, thereby reducing oxygen and blood flow, which should make them grow weaker.

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

"This tells us that oxytocin's role, although significant, is not the only thing contributing to how this vital muscle contracts during labor. ... How does this powerful muscle carry on working against the odds?" In the heart, something called "hypoxic preconditioning" helps cells change in such a way that they protect the heart from more serious drops in oxygen.

This can be lifesaving, and similarly, the researchers saw in the lab that sample uterine tissue reacts to the repetitive dip in oxygen and blood supply of preceding contractions by triggering what the Liverpool researchers are calling "hypoxia-induced force increase (HIFI)," they report in the Proceedings of the National Academy of Sciences.

And once the HIFI mechanism is triggered, it can last for hours, which may seem like cruel torture to the women in labor but could be protective.

"This uterine trigger could be key to resolving issues of prolonged labor, as well as the increasing number of births that result in emergency caesarean," Wray says.

(Meanwhile, boys and girls have different spines from birth.)

This article originally appeared on Newser: Scientists Learn Why Labor Contractions Grow Stronger

More From Newser