Birth weight and postnatal growth in preterm born children are associated with cortisol in early infancy, but not at age 8 years.

Background: Preterm birth has been associated with altered hypothalamic-pituitary-adrenal (HPA-) axis activity as well as cardiometabolic diseases and neurodevelopmental impairments later in life. We assessed cortisol from term age to age 8 y in children born preterm, to explore the development of HPA-axis activity in association with intrauterine and early-postnatal growth until 6 mo. corrected age.

Methods: In 152 children born at a gestational age ≤32 wks. and/or with a birth weight ≤1,500g, random serum cortisol was assessed at term age (n=150), 3 mo. (n=145) and 6 mo. corrected age (n=144), and age 8 y (n=59). Salivary cortisol was assessed at age 8 y (n=75): prior to bedtime, at awakening, 15min after awakening, and before lunch. Cortisol was analyzed in association with birth weight-standard deviation score (SDS), being born small for gestational age (SGA), and combinations of intrauterine and postnatal growth: appropriate for gestational age (AGA) with or without growth restriction (AGA GR+ or AGA GR-) at 6 mo. corrected age, and SGA with or without catch-up growth (SGA CUG+ or SGA CUG-) at 6 mo. corrected age. Cross-sectional associations at all time points were analyzed using linear regression, and longitudinal associations were analyzed using generalized estimating equations.

Results: Longitudinally, birth weight-SDS was associated with cortisol (β [95%CI]): lower cortisol over time was seen in infants with a birth weight ≤-2 SDS (-50.69 [-94.27; -7.11], p=0.02), infants born SGA (-29.70 [-60.58; 1.19], p=0.06), AGA GR+ infants (-55.10 [-106.02; -4.17], p=0.03) and SGA CUG- infants (-61.91 [-104.73; -19.10], p=0.01). In cross-sectional analyses at age 8 y, no associations were found between either serum or salivary cortisol and birth weight-SDS, SGA-status, or growth from birth to 6 mo. corrected age.

Conclusion: In children born preterm, poor intrauterine and postnatal growth were associated with lower cortisol in early infancy, but not at age 8 y. Even though HPA-axis activity no longer differed between groups at age 8 y, or differences could not be confirmed due to attrition, it is unknown whether the differences found in early infancy could attribute to increased health risks later in life.