Human newborn behavior during exposure to maternal and other odors

Olfaction appears to play a pivotal role in immediate postnatal interactions between mammalian mothers and their offspring. The present study examined: (1) whether and how the newborn human infant and its mother communicate by means of olfaction; (2) the role of amniotic fluid (AF) and breast odors in such communication; (3) olfactory learning by neonates. An undisturbed communication may play an important role in the initiation and maintenance of breastfeeding. The newborns of studies I-V were uninfluenced by pharmacological pain relief given to the mother.

Newborn babies seek and locate the mother's breast within the first hour after birth and prefer an unwashed breast to the washed alternative, indicating that olfaction is implicated in breast seeking behavior (I). Other senses (e.g. vision, touch, temperature sensitivity) may also help the baby to locate the nipple. To determine more precisely the role of olfaction, a cotton pad bearing the mother's breast odor was placed 17 cm in front of the baby who was laying prone on a warm bed. The breast odor elicited crawling movements towards the cotton pad and stereotyped prefeeding behaviors that are typically observed when babies are placed on their mother's chest. These findings indicate that smell, in the absence of other maternal cues, helps guide the baby to the nipple and evokes the behavioral preludes of breastfeeding (V).

Amniotic fluid odor is attractive for mammalian neonates. The current study has shown that newborns prefer to suck from a breast treated with AF rather than a naturally smelling breast during the first postnatal hour, possibly reflecting prenatal learning (II). Within several days after birth, the AF preference was replaced by a preference for the natural breast odor, suggesting postnatal olfactory learning (III).

Movement of the hand to the mouth and finger sucking are components of the prefeeding behavior displayed immediately after birth. This behavior is disturbed when AF was washed off the baby before it was given to the mother (II). Washing may therefore disrupt prefeeding behavior and proper attachment to the nipple, and related studies have shown that problems with nipple attachment are associated with early breastfeeding failure.

Odors can influence behavior during stress. Newborns who were separated from their mother during the first postnatal hours were exposed to either AF odor or maternal breast odor presented on cotton pads. AF soothed newborns, whereas breast odor elicited increased crying relative to control trials (IV). The calming effect of AF might reflect prenatal exposure to that scent. Maternal breast odor may have signalled the presence of food, and the babies became frustrated/agitated when they could not reach the source.

Immediate postnatal olfactory learning is shown in several mammals. The capability for early postnatal olfactory learning was studied in Caesarean section delivered babies, who during the first postnatal hour, were exposed for 30 min to an artificial odor (cherry or passion fruit) (VI). Caesarean section delivered babies demonstrated such olfactory learning at testing about three days later, but only if the mothers had experienced uterine contractions before the operation. The norepinephrine levels in the umbilical artery correlated positively to learning. This learning may depend upon locus coeruleus activation and/or the catecholamine surge, and is further evidence that uterine contractions contribute to neonatal adaptation.

In summary, the newborn and its mother communicate by means of olfaction. Several common hospital care routines such as washing the baby immediately after birth and the mother's breasts before feeding interfere with this communication and should be avoided until we know more about its clinical significance.

List of scientific papers

I. Varendi H, Porter RH, Winberg J (1994). "Does the newborn baby find the nipple by smell? " Lancet 344(8928): 989-90
https://pubmed.ncbi.nlm.nih.gov/7934434

II. Varendi H, Porter RH, Winberg J (1996). "Attractiveness of amniotic fluid odor: evidence of prenatal olfactory learning" Acta Paediatr 85(10): 1223-7
https://pubmed.ncbi.nlm.nih.gov/8922088

III. Varendi H, Porter RH, Winberg J (1997). "Natural odour preferences of newborn infants change over time. " Acta Paediatr 86(9): 985-90
https://pubmed.ncbi.nlm.nih.gov/9343280

IV. Varendi H, Christensson K, Porter RH, Winberg J (1998). "Soothing effect of amniotic fluid smell in newborn infants. " Early Hum Dev 51(1): 47-55
https://pubmed.ncbi.nlm.nih.gov/9570031

V. Varendi H, Porter RH (2001). "Breast odour as the only maternal stimulus elicits crawling towards the odour source. " Acta Paediatr 90(4): 372-5
https://pubmed.ncbi.nlm.nih.gov/11332925

VI. Varendi H, Porter RH, Winberg J (2001). "The effect of labor on olfactory exposure learning within the first postnatal hour." Behavioral Neuroscience (Submitted)