Display omitted ► CE-LIF/MS n of oligosaccharides in breast milk and corresponding baby feces in time. ► No major changes in presence of HMOs in breast milk during 6 months. ► Acidic or neutral fecal HMOs during first few months (stage 1), dependent on milk. ► Joint HMO and blood group specific oligosaccharides (A, B, H(0)) dominant (stage 2). ► No HMOs and blood group oligosaccharides when solid food is introduced (stage 3). The characterization of oligosaccharides in the feces of breast-fed babies is a valuable tool for monitoring the gastrointestinal fate of human milk oligosaccharides (HMOs). In the present study we monitored fecal oligosaccharide profiles together with the HMO-profiles of the respective breast milks up to six months postpartum, by means of capillary electrophoresis-laser induced fluorescence detection and mass spectrometry. Eleven mother/child pairs were included. Mother’s secretor- and Lewis-type included all combinations Le(a−b+), Le(a+b−), Le(a−b−). The fecal HMO-profiles in the first few months of life are either predominantly composed of neutral or acidic HMOs and are possibly effected by the HMO-fingerprint in the respective breast milk. Independent of the initial presence of acidic or neutral fecal HMOs, a gradual change to blood-group specific oligosaccharides was observed. Their presence pointed to a gastrointestinal degradation of the feeding-related HMOs, followed by conjugation with blood group specific antigenic determinants present in the gastrointestinal mucus layer. Eleven of these ‘hybrid’-oligosaccharides were annotated in this study. When solid food was introduced, no HMOs and their degradation- and metabolization products were recovered in the fecal samples.