Introduction
The interest in the function of the hypothalamic-pituitary-adrenal (HPA)-axis of pregnant women is growing, as evidenced by the evolving literature revealing the influence of maternal stress on the offspring´s physical and mental health outcomes 1-5.
However, the evidence regarding whether prenatal maternal cortisol levels are the sole or main mediating link is conflicting6. It also seems that foetal vulnerability to high maternal cortisol levels varies across the gestational phases4, 6-8.
Research regarding the impact of cortisol in the mediation of maternal stress upon the foetus is complicated by the fact that the HPA-axis activity shows substantial diurnal variability 9 in addition to individual differences. Single cortisol measures in plasma or saliva may therefore not sufficiently reflect the overall long-term biological activity of cortisol. Analysis of hair cortisol concentrations (HCC) is used to overcome such limitations10-12, where one cm of hair corresponds to about one month´s cortisol accumulation13. This method has been validated against cortisol in saliva across the pregnancy and postpartum periods and has proved to be a reliable metric of HPA activity, enabling estimation of integrated cortisol release14.
Reports on HCC throughout pregnancy have revealed varying results, probably due to the different analysis methods used11, 14-18. The research focus has also been wide-ranging and has shown several factors of importance for HCC, including season, obesity11,19 and delivery mode 11. In addition, some 18-19 researchers have noted a relation between psychosocial and lifetime stress exposure on HCC during pregnancy, while others 20 have concluded that neither psychological distress, nor chronic stress or psychiatric symptoms had any impact.
Higher cortisol levels have been reported in primiparae versus multiparae21-22, probably related to more pregnancy distress 23. However, Federenko et al. in 2006 could not detect any effect of parity on cortisol levels22.
HCC covering single-month periods may give more precise and more coherent results when investigating relations to childbirth and neonatal outcomes, but to our knowledge, no reports of measurements conducted on a monthly basis are available regarding HCC during pregnancy and postpartum. We hypothesised that there may be variations in cortisol hair levels during pregnancy and postpartum, possibly influenced by parity. Hence, the purpose of this study is to add to the knowledge of normal HPA-axis functioning during pregnancy and postpartum by determining HCC levels on a monthly basis. A second aim was to explore further differences in HCC levels associated with parity during pregnancy.