For fertilization to occur - for a spermatozoia to unite with an ovum- the m man must ejaculate into the woman's vagina . This expulsion occurs during sexual relations , when a man achieves an intense moment of pleasure called orgasm. However , an ejaculation does not necessarily lead to fertilizer , since the period of fertility lasts only a few days in the ovarian cycle . if the ovum is not fertilized during this short period v, it degenerate and is eliminated with the menstrual flow