The invention relates to a method for the Raman spectroscopic, in ovo sex determination of fertilised and hatched birds' eggs (1), wherein the embryo, including the extra-embryonic structures, can move in the egg, and is not yet attached to the shell at the time of measuring. In addition, the following steps are carried out: monitoring the time course of the hatched egg until forming at least one recognisable blood vessel (21); creating a hole (2) in the shell in the region near to the attached bloody vessel, using a hole-generating unit; finding the blood vessel forming in the egg, using a vision system (19, 13) and a coaxial or lateral illumination with light (10a) in the visible wavelength range; positioning at least one blood vessel in the laser focus of a laser source (3), either by moving the egg or moving a lens (6) of a device (5) for introducing the laser light (3a), and detecting the Raman scattered radiation (7); registering the Raman scattered radiation of the irradiated blood vessel using the device for introducing the laser light, and for detecting the Raman scattered radiation, wherein, during the measuring process, a movement of the blood vessel out of the focus can be avoided by tracking using the vision system; evaluating the Raman scattered radiation in an evaluation unit; determining and displaying the sex of the embryo in the bird's egg.