Background  Functional and structural skin adaptation is a dynamic process which starts immediately after birth in humans and in mammalian skin in general. This adjustment to the extrauterine dry environment is accomplished in the first year of postnatal life of humans.

Objectives  To assess the dynamic changes in vivo after birth in the molecular composition and skin physiology parameters compared with older children and adults.

Methods  The molecular composition of the stratum corneum (SC) and the water profile were investigated noninvasively by in vivo Raman confocal microscopy as a function of depth. Functional parameters including transepidermal water loss (characterizing epidermal permeability barrier), capacitance (as an indirect parameter for SC hydration) and skin surface pH were assessed noninvasively. The measurements were performed in 108 subjects divided into six age groups: full-term newborns (1–15 days), babies aged 5–6 weeks, babies aged 6 ± 1 months, children aged 1–2 years, children aged 4–5 years and adults aged 20–35 years.

Results  We showed that skin acidification is still under development during the first weeks of life. While the basal epidermal barrier is competent immediately after birth, the SC is less hydrated in the first 2 weeks of postnatal life. Similar continuous decreasing water content towards the surface for all age groups was observed, whereas this gradient was lower for the newborns. Dynamic changes in the amounts of the natural moisturizing factor constituents were revealed in the period of infancy.

Conclusions  We demonstrated the relation of formation of an acidic pH as well as underlying mechanisms in the induction of a fully hydrated SC over the first weeks of human life as a dynamic functional adaptation.