Abnormal preimplantation embryonic development Formation of multiple pronuclei during fertilization 2021-04-25T23:27:41Z Three pro-nuclei (3PN) prevalence among all pregnancies has been estimated to be approximately 1% to 3%, whereas it accounts for 15% to 18% of cytogenetically abnormal cases among spontaneous abortions. Pronuclei formation is a critical process during fertilization. Normally, there are two pronuclei (2PN), including the paternal pronucleus and the maternal pronucleus, in the zygote after fertilization. It is generally accepted that multiple pronuclei (MPN) formation is due to the abnormal extrusion of the second polar body or to abnormal fertilization with multiple sperm. The MPN therefore can cause infertility and recurrent failure of IVF/ICSI.