You're no doubt familiar with the image of a baby sucking his thumb, but unborn babies in the later stages of development do it too. In fact, new research reveals that fetuses actually open their mouths in anticipation of a good thumb suck, which may point to better ways of tracking fetal development and understanding the limitations of babies born prematurely.

In a finding out of the University of Durham, researchers used 4D ultrasound scans to track the movements of 15 fetuses during development in the third trimester. These 4D scans offer three-dimensional footage with the added benefit of realtime movement — basically allowing researchers to closely track the various maneuvers of a wee fetus.

The scans revealed that fetuses in the earlier stages of the third trimester (around 24 weeks) typically touched only the upper and sides of their heads. As development progressed, the fetuses gradually started to touch more sensitive facial regions, including the mouth. And by 36 weeks (or 9 months), they actually opened their mouths prior to touching them. In other words, the babies had learned to anticipate contact between hand and mouth rather than simply react to it.

Experts behind the research suggest that it might be accompanied by several important implications. For one, zeroing in on a timeframe wherein babies develop the ability to anticipate touch could help with the care-taking of premature infants — knowing, for example, at what stage in development a baby will be primed for feeding. Study authors also suspect that the finding could offer a means of evaluating and tracking healthy fetal development, though that depends on future research.