The present invention relates to a method and a system for producing a mammalian pre-embryo and a stem cell having a better quality than prior art methods. The system comprises means for obtaining a mammalian oocyte, and means for obtaining a mammalian spermatozoa, and an apparatus having at least two separate air-tight chambers, for which the oxygen tension of one chamber may be changed independent of the oxygen tension of the other chamber, said at least two separate air-tight chambers constitute a main chamber and at least one residence chamber. The method for in vitro producing a mammalian pre-embryo comprising the steps: a1) providing a mammalian oocyte, a2) providing a mammalian spermatozoa, b) culturing the oocyte and the spermatozoa, c) fertilizing the oocyte with the spermatozoa obtaining a fertilized oocyte, and d) allowing cell-division of the fertilized oocyte obtaining a multicellular pre-embryo wherein at least one of the steps a1) or a2) is conducted at an oxygen tension below 15%, or e) allowing cell-division of the fertilized oocyte obtaining a multicellular pre-embryo, wherein the culture is performed at an oxygen tension allowing cultivation of the cells and wherein at least one of the steps comprises a change in the oxygen tension. Stem cells are produced from the multicellular pre-embryo.