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Factors such as maternal nutrition, stress, and mental health during pregnancy can influence the formation and function of fetal biological systems, such as the hypothalamic-pituitary-adrenal (HPA) axis —this is referred to as prenatal programming and is an important factor in disease risk.
The HPA axis is involved with stress response and production of cortisol. Measures of salivary cortisol are frequently used to understand early life HPA axis function. Measurements of cortisol reactivity taken closest to birth have potential to reveal how prenatal maternal exposures affect offspring physiology, as confounding postnatal exposures can be minimized. However, there is a lack of data on young or healthy infants due to challenges of measuring infant reactivity, such as frequent sleeping and eating, and difficulties recruiting participants willing to engage in “stress-reactivity” studies soon after birth. Subsequently, there is a lack of normative ranges for salivary cortisol values for infants under age two months, and existing data stems from very small samples. Our study sought to determine whether a challenging social and physical stress paradigm would activate HPA axis reactivity, thus providing information for basal and reactivity salivary cortisol concentrations in newborns.
Method
Our sample is from the Stress, Eating, and Early Development study (SEED), a longitudinal cohort of 180 mother-child dyads (84 boys, 96 girls) from varied socioeconomic backgrounds within the San Francisco Bay area. The cohort includes 69 African American (38.2%), 58 Hispanic/Latina (32.0%), 26 Caucasian (14.6%), 21 Other/Multiracial (11.8%), 5 Asian (2.8%), and 1 Pacific Islander (0.6%) mothers who had pre-pregnancy body mass index (BMI) of 25-40 and household incomes less than 500% of the federal poverty level.
Mother-child pairs were visited when the infant was 30 days old (M = 36.22, SD = 16.29). Study visits included administration of the Brazelton Newborn Behavioral Observation (NBO) procedure, during which infants engaged in developmentally appropriate challenges and anthropometric measuresments (length, weight, skin fold thickness). Salivary cortisol was collected at baseline (A) and 15 minutes after task completion for reactivity (B). Values outside of 3 SD from the mean, children taking steroid medications, and implausible values over 100 nmol/L were removed, and values were log transformed to create normally distributed reactivity scores.
Results
Raw value ranges for A and B cortisol measurements are 0.21 – 41.01 nmol/L (N = 132, M = 9.52, SD = 8.447) and 0.75 – 82.05 nmol/L (N = 128, M = 16.07, SD = 15.47), respectively. 59.3% of the sample increased in cortisol concentration from A to B (d = 0.526). There were no significant correlations between log-cortisol reactivity and child sex, birth weight, gestational age, or 5 minute APGAR scores. Analyses examining prenatal stress effects on cortisol reactivity, as well as the relation between cortisol reactivity and infant behavior and temperament, will be reported in May.
Conclusions
Reference ranges for baseline and cortisol reactivity are important for future research into HPA axis functionality in infancy and will advance prevention and intervention science.