With the technique of 4D ultrasound exams today we have access to clearer images of intrauterine life that allow the detailing of behavior and the study of facial expressions of fetuses. Currently, it is common to see images of the beginning of the second trimester of pregnancy showing behaviors such as sucking the finger, expelling the tongue, yawning, and even showing facial expressions of crying and laughter, even though it is not possible to conclude that they are associated with suffering or joy.

Recently, researchers observed that the spatial and temporal characteristics of fetal movements were not uncoordinated and appeared to have an advanced level of motor planning. From this discovery they decided to study the movements of twin fetuses, because until then the touch between them was attributed to reflex behavior, an unintended automatic behavior.

The research showed that the touches directed at the co-twin were longer and increased the frequency as the pregnancy progressed in relation to the self-directed touches, which were shorter and decreased over time. This survey must be replicated with a larger number of participants in order to confirm the results. But if confirmed, this difference suggests that the movements are not accidental, but that there is an intention to reach the co-twin and, therefore, a form of initial social interaction between siblings still in the womb!