Stress responses at birth: determinants of cord arterial cortisol and links with cortisol response in infancy

Objective To investigate (A) the determinants of infant stress response at delivery and (B) test the hypothesis that stress at birth, as reflected by cord arterial cortisol, influences cortisol response to vaccination at two months. Design Prospective observational study. Setting Tertiary referral maternity hospital. Population One hundred and seventy-two primiparous women with uncomplicated singleton pregnancies. Methods Women were recruited antenatally. At birth, cord arterial blood and obstetric data were collected. Saliva was collected from infants immediately before and after vaccination at two months. Cortisol was analysed from cord blood and saliva by radio-immunoassay. Main outcome measures Stress response at birth, as demonstrated by cord arterial cortisol; association with saliva cortisol response to vaccination at two months. Results Cord arterial cortisol varied with mode of delivery, combined spinal/epidural use and pH. Salivary cortisol response at two months correlated with cord arterial cortisol (r= 0.24, P < 0.05). Infants with the highest and lowest cord arterial cortisol had markedly different cortisol responses at two months (P= 0.017). These groups had different modes of delivery with caesarean rates of < 8% in the high cortisol response group and 83% in the low cortisol response group (P < 0.0001). Conclusion Babies born vaginally mount greater cortisol responses at birth than those delivered by caesarean section. Stress at delivery may influence the infant HPA axis response for up to two months.