The following morning after oocyte retrieval, i.e. 16-20 hours after insemination the fertilization check is performed. The embryologists check under the microscope how many oocytes have been fertilized normally and isolate unfertilized or abnormally fertilized oocytes. Normal fertilization is verified by the presence of 2 pronuclei (2PN) and 2 polar bodies.
The chromosomes of the spermatozoon and the oocyte have been condensed and are enveloped by a membrane, giving rise to the two pronuclei (1 male and 1 female). At the same time the second polar body is extruded marking the completion of meiosis II. The first cells of the new embryo is now called zygote and has the same size and shape as the oocyte.