Mammalian fetuses are able to sense flavor cues transferred through the maternal diet in the amniotic fluid. Such prenatal chemosensory experience engages fetal learning and memory that extends into the postnatal environment. Despite its importance, we know very little about what human fetal chemosensory abilities mean for postnatal food-related behaviors. The effects of prenatal flavor exposure in humans have been investigated on infant behaviors, however, longitudinal investigation of reactions from fetal to neonatal life has to date been lacking. To address this, we assessed fetal and neonatal olfactory responses to flavors transferred from the maternal diet in the last trimester of pregnancy. 32 fetuses/neonates (kale flavor exposure, n = 14, carrot flavor exposure: n = 18) were involved in this study from 36 weeks until the first postnatal month. At 36 weeks, fetal facial reactions were measured via a 4D ultrasound scan and a reliable coding system (FACS; Ekman & Friesen, 1978) after maternal ingestion of one flavor capsule. After 36 weeks scan, mothers consumed the same flavor capsule repeatedly for 3 consecutive weeks. In the first postnatal month (mean age: 3.06 weeks), neonatal facial reactions to kale, carrot and control odor were analyzed using FACS. Repeated flavor exposure during the last 3 weeks of pregnancy resulted in an increased frequency of laughter-face-like reactions from fetal to neonatal stage when exposed to non-bitter-tasting flavor. More interestingly, a changing pattern from cry-face-like to laughter-face-like reactions from fetal to neonatal stage occurred when exposed to bitter-tasting flavor in utero. This study has important and novel implications in sensory science by providing a window into the chemosensory world of the human fetus, which could reduce the likelihood of fussy eating when introducing solid foods conveying bitter taste.