Bibian van der Voorn

45 INTERPRETATION OF GC LEVELS IN NEONATAL HAIR 28, the anagen hair converts to telogen hair via a catagen phase 26 . Hair growth, as well as the conversion to more mature hair, is region-specific, and dependent on several biochemical and individual variations 26-28 . Whether hair in the anagen phase already contains GCs, or whether the accumulation of GCs occurs at a later phase, is unknown. Therefore, although it is thought that neonatal hair reflects at least the third trimester of pregnancy 26 , the true time window which is represented by GCs measured in hair is not known. Since perinatal infection and mode of delivery also appear to influence hair GC levels, it is likely that the last stages of pregnancy have a significant contribution to GC levels measured in hair. Future studies should include measurements of growth velocity of neonatal hair. Our study showed significantly increased GC levels in neonatal hair compared to maternal hair at the OPV (44±11 days postpartum), although a decrease between birth and the OPV was observed in cortisol levels. This suggests that GC levels at the OPV represent a combination of intra- and extrauterine influences, supported by our finding that GC levels at the OPV are still associated with several perinatal factors. However, due to the biochemical and individual variations in hair growth 26-28 , and since hair was only measured twice in this study, the contribution of intrauterine and extrauterine influences on hair GC levels at the OPV is unknown. We recommend to assess at which point in time intra-uterine factors are no longer related to hair GC levels, since this might provide a clear view of early life influences on HPA axis development. Since hair GC levels appear to be moderately stable in the second half of the first year of life 28 , intrauterine influences on hair GC levels most likely disappear within the first 6 months. Our study has several strengths and limitations. First, GC analyses were performed using LC-MS/MS, which has high sensitivity. Hoffman et al. 18 measured hair cortisol levels with an immunoassay, which might explain the fact that they did not find an association between maternal and neonatal hair cortisol levels, and reported maternal and neonatal cortisol levels which were much higher compared to our results, since immunoassay are more sensitive to cross-reactivity than LC-MS/MS 30,31 . Cross-reactivity is particularly important to take into account when researching newborns, as they have high concentrations of (precursors of) sex steroids and GCs 5 , which are partly of maternal origin. Additionally, we measured cortisone as well as cortisol, which providess valuable knowledge due to the conversion of cortisol to cortisone by placental 11βHSD2 8 . Our database also allowed us to analyze a wide range of pre- and perinatal factors. One of the limitations of our study is that the participants might not represent a normal population, since the participants in our study had to be hospitalized >72h. Additionally, there might be selection bias at the OPV measurements due to losses to follow-up, although the number of participants