Babies born by cesarean section have a 50 higher chance of developing asthma than those born naturally, according to a new study in Norway.

While asthma is 40 percent more common in C-section babies than those born naturally, the risk goes up 60 percent for those born by emergency C-section, according to a news release from the authors of the study.

It also found that babies who needed forceps or a vacuum to make it out into the world had a 20 percent greater chance of asthma.

Dr. Mette Christophersen Tollanes, the lead author, said it may be that babies born this way miss being exposed to certain bacteria from their mothers, so their immune systems don't develop in the same way.

Another theory is that babies who go through the birth canal experience more stress hormones and have their chests squeezed, which helps remove amniotic fluid.

The news release did not address if the study considered that pregnancies that require emergency C-sections could have other problems that would make asthma more common.

The work was based on more than 1.7 registered births in Norway.