Each month, a woman naturally produces a mature oocyte from a pre-ovulatory follicle, which is then ovulated. Fertilization occurs when this oocyte meets a sperm cell. It involves highly complex interactions, requiring the maturation of sperm in the female reproductive tract and the ability of the mature oocyte to recognize and bind with the sperm, allowing fusion.  

Once the sperm has penetrated the oocyte, two pronuclei quickly appear—the female pronucleus and the male pronucleus, each containing the chromosomal material from both individuals. The recognition and fusion of maternal and paternal chromosomes result in the formation of the zygote, the first stage of embryonic development.