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This study was aimed at characterizing the level of specificity of the human newborn's response to an odor experienced in utero. Three groups of breast-fed infants and 3 groups of bottle-fed infants were examined on Postnatal Day 3 for their differential head-turning response when exposed to paired-choice tests contrasting the odors of either familiar (f) amniotic fluid (AF) or nonfamiliar (nf) AF or either of these AF odors and a control (C) stimulus. In fAF versus nfAF tests, the infants oriented preferentially to the odor of fAF, regardless of their feeding regimen (i.e., of their postnatal reexposure to AF-like cues in maternal milk). The fAF or nfAF versus C tests showed that this response pattern was caused by a true positive orientation toward fAF and not by avoidance from nfAF odor. This highly selective neonatal response to fAF odor is consistent with the hypothesis that the human fetus can detect and store the unique chemosensory information available in the prenatal environment and that this information becomes coupled with positive control of behavior.

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.

Behavioral responses of 3- to 4-day-old newborns to the odors of various human milk (HM) and formula milk (FM) were examined in paired-choice tests. When both stimuli were nonfamiliar, breast-fed, as well as bottle-fed, infants oriented their head and mouthed more vigorously to HM than to FM. When breast-fed infants were exposed to nonfamiliar HM along with the familiar FM, their head-turning responses were undifferentiated although they mouthed more frequently to the human stimulus. When nonfamiliar HM and familiar FM were equalized in intensity, nonfamiliar HM again elicited more head orientation and mouthing responses. These results demonstrate that the odor of HM is more attractive to human newborns than FM and that this preference is independent of postnatal feeding experience.

Two-day-old newborns were videotaped during a double-choice test contrasting the odours of their amniotic fluid (AF) and of a control stimulus. To control for early motor asymmetries, the lateral position of both stimuli was balanced both between and within subjects. On average, neonates oriented their nose for a significantly longer duration toward the odour of AF. Regardless of the nature of the stimulus, neonates also evinced a marked head-turning bias to the right side. The nature of the odour stimulus and side of presentation interacted so that infants turned their nose longer to the AF odour when it was presented from the right side. These data indicate that newborns can detect the AF odour, and that they remain attracted towards it for at least 2 days after birth.

The introduction of a pleasant odor in the incubator is of therapeutic value in the treatment of apneas unresponsive to caffeine and doxapram.

The head‐orientation response of 2‐ and 4‐day‐old bottle‐feeding neonates was studied in paired‐choice odor tests. Three tests were conducted at Days 2 and 4 after birth to assess the development of the relative response between two salient odors from the prenatal and postnatal environments: (a) amniotic fluid (AF) versus formula milk (FM), (b) FM versus control stimulus (distilled water), and (c) AF versus control stimulus. At both ages, AF and FM elicited positive orientation when presented simultaneously with the control stimulus, indicating that both odors were detectable and attractive to the infants. However, when AF and FM were presented concurrently, the infants expressed significantly longer orientation response toward AF odor than toward FM odor at the age of 2 and 4 days. Within the first 4 days of life, bottle‐feeders thus display olfactory preference for a prenatal substrate over a postnatal substrate to which they were recurrently exposed in the feeding situation. © 1998 John Wiley & Sons, Inc. Dev Psychobiol 33: 133–145, 1998

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