(As yet) unsolved questions about amniotic fluid-borne flavours and their perception by the human fetus. Reply to the letter by Dr J. G. Alves in Psychological Science

We thank Dr Alves for his comments on our paper entitled “Flavor Sensing in Utero and Emerging Behaviors in the Human Fetus” (Ustun et al., 2022). As we acknowledge and Dr Alves highlights, the behavioural responsiveness of the human fetus to chemosensory stimuli delivered in utero has so far been rarely investigated in utero, contrary to rat or ovine fetuses, which behavioural and psychophysiological reactions could be more extensively assessed using chemostimuli applied directly in the womb (e.g., Robinson et al., 1995; Schaal et al., 1991; Smotherman & Robinson, 1987). Rather most studies rely on a posteriori evidence with infants being re-exposed to chemostimuli which they could “only” have experienced prenatally. Therefore, many of the biological and psychobiological issues raised by Ustun et al.’s study need further work. Specifically, on 1) chemoreceptor system(s) and 2) brain structures and cognitive processes involved, on 3) the nature, kinetics and pathways of exogenous chemostimuli that reach fetal chemoreceptors, and on 4) the nature and timing of fetal responses. In this letter, we reply to Dr Alves’s comments.