The effect of labor on olfactory exposure learning within the first postnatal hour.

Thirty-one neonates delivered by cesarean section were exposed to an odor for 30 min shortly after birth. Fifteen births had uterine labor contractions before delivery; 16 were without contractions. All babies were later tested (median age = 80 hr) for their responses to the familiar exposure odor and a novel odor presented on either side of the face. Overall, the babies spent more time turned toward the exposure odor than toward the novel scent. Babies in the labor condition, but not those bom without labor, displayed a significant preference for the exposure odor. Norepinephrine (NE) levels were higher in babies who oriented preferentially toward the exposure odor. Brief exposure immediately after birth is sufficient for the development of olfactory learning. Heightened learning by neonates from births with contractions may reflect locus coeruleus and NE activation. Olfactory learning may therefore be particularly efficient shortly after birth.