The present invention provides methods to determine the sex of an embryo by detecting the presence of a protein associated with the Y chromosome in a sample of culture medium obtained by sampling a culture medium containing the embryo. The present invention further provides methods to identify a protein associated with the Y chromosome as a protein present in a sample of culture medium obtained by sampling a culture medium containing a male embryo, but absent in a sample of culture medium obtained by sampling a culture medium containing a female embryo.