When a pregnant woman experiences stress, her unborn fetus uses its left hand more, new research finds.

A mother’s stress therefore affects her fetus and may impact the child’s development after birth and throughout his or her life. The findings are published in the journal Laterality: Asymmetries of Body, Brain and Cognition.

“While we observed a higher degree of left-handed behavior in the fetuses of stressed mothers than had been expected, we are not saying that maternal stress leads to a child becoming left-handed after birth, as there could be a number of reasons for this,” lead author Nadja Reissland of Durham University’s Department of Psychology said in a press release.

PHOTOS: Prehistoric Moms Had Their Hands Full

“The research does suggest, however, that a fetus can detect when a mother is stressed and that it responds to this stress,” she added.

Reissland and her colleagues used 4-D ultrasound scans to observe 15 healthy fetuses at four different stages between 24 and 36 weeks of pregnancy. During this time, the researchers also asked the mothers how much stress they had experienced in the four weeks between each of the scans.

Reissland and her team found that the more stress mothers reported, the more frequently fetuses touched their faces with their left hands. Since right-handedness is more common in the general population, the scientists expected to see more of a bias towards right-handed movements in the fetuses as they grew older.

This did happen, but not when the moms were stressed out. That’s when the fetuses exhibited the tendency toward left-handedness.

VIDEO: Design Your Own Baby: Patent Granted

“This suggests maternal stress could be having on effect on the child’s behavior in the womb and highlights the importance of reducing maternal stress in pregnancy,” Reissland said. “Such measures may include increased emphasis on stopping stressful work early, the inclusion of relaxation classes in pre-natal care and involvement of the whole family in the pre-natal period.”

In this case, the cause is more worrisome than the fetus' reaction. When people are stressed out, they tend to produce extra cortisol, a potent stress hormone.

Reissland and colleagues say that future research might then pay more attention to how cortisol levels impact fetuses and longer-term human development.

Photo: Human embryo. Credit: Ed Uthman, Wikimedia Commons