(Courtesy Nadja Reissland/University of Durham)

Practice for all those baby smiles starts early, according to a new study. In-the-womb early.

The study of sonograms found that fetal facial expressions start ramping up during the second half of pregnancy. And they're not just cute; they're a crucial consequence of brain development.

"As the brain matures, more and more movements come together," said Nadja Reissland, a researcher at the University of Durham in the United Kingdom and lead author of the study published in the April issue of the journal Physiology and Behavior. "In the end, [babies] seem to be ready to come out of the womb and have emotional expressions."

The right side of the brain controls the left side of the face, and it's also thought to play a leading role in the expression of emotions. After analyzing 176 mouth movements, Reissland found that the majority - 64 percent - occurred on the left side of the face thanks to signals from the right side of the brain.

It makes sense, according to Reissland, because newborn babies have to bond with their parents. And because they can't talk, they make faces.

(Image Credit: PLoS One)

Reissland previously caught fetuses yawning in the womb, but not because they're bored (even though nine months is a long time to be curled up in the dark). She said yawning, too, is a byproduct of brain development. It's hard work!