Fetuses show various facial expressions, such as blinking, yawning, sucking, mouthing, tongue expulsion, scowling, and smiling, assessed by four-dimensional ultrasound from the early second trimester of pregnancy. Moreover, some fetuses demonstrate emotion-like behaviors as noted in infants, children, and adults. Selective fetal responses to stimuli indicate a high degree of brain development, which is reflected in the facial reactions. The fetal face with its movements and expressions can mirror the fetal brain function and development during different stages of the fetus in utero. The fetal face might represent the key to opening the box containing the secrets of fetal brain function and development. Four-dimensional ultrasound may be an important modality in current and future research on fetal facial expressions and assist in the evaluation of the fetal brain function.