Neonatal responsiveness to the odor of amniotic and lacteal fluids: a test of perinatal chemosensory continuity.

The head-orientation response of 2- and 4-day-old breast-feeding neonates was studied in paired-choice odor tests. Three tests were conducted on day 2 (amniotic fluid [AF] versus Colostrum; AF versus Control; Colostrum versus Control) and on day 4 (AF versus Milk; AF versus Control; Milk versus Control). At 2 days, both AF and Colostrum elicited positive orientation when presented simultaneously with the control stimulus, indicating that both odors were detectable to the infants. However, no differential responses were noted when AF and colostrum were presented concurrently, suggesting that both of these substrates were treated as similar sensorily and/or hedonically. On day 4, the odors of AF and transitional milk elicited attraction responses when presented in competition with a control stimulus. When the odor of milk was presented simultaneously with the odor of AF, the former elicited longer head orientation. Thus, within the first 4 days of life olfactory selectivity changes from a null preference between cues carried in AF and in colostrum to a positive preference for cues carried in postamniotic odors, that is, breast milk. An additional experiment indicated that 3-day-old neonates orient longer toward the odor of their own AF than toward the odor of alien AF, showing that prenatal odors elicit selective responding for some time after birth. Altogether these results were interpreted as supporting the hypotheses that prenatal experience might influence the earliest odor preferences in the breast-feeding human neonate and that these preferences rapidly evolve according to postnatal experience.