Fertilisation is defined as the fusion of the sperm nucleus with the ovum nucleus. For fertilisation to occur, sperm must be deposited in the vagina no earlier than three days before ovulation or within one day after ovulation. This short 'window of opportunity' is because the sperm and the ovum have only limited lives, and they both soon die if they do not meet and fuse in this period. Following ovulation, the ovum is picked up by the fimbriae of the fallopian tube on the same side of the body as the ovary that released the ovum. The ovum will remain in the fallopian tube, alive and fully functioning, for only about 12 to 24 hours. Sperm can live longer in the female reproductive system, up to 72 hours, but most die before this time.
Once deposited within the vagina, the sperm swim through the cervix and into the uterus, and then up into the fallopian tubes (look back at Figure 3.3 to remind yourself of the anatomy). Fertilisation of the ovum occurs in the fallopian tube. The movement of sperm on this long journey is helped by muscular contraction of the walls of the uterus and the fallopian tubes. Sperm can swim several millimetres (mm) in a second, so it only takes them about 15 minutes to get into the fallopian tubes, but millions die along the way.
Only one sperm will succeed in fertilising the ovum (Figure 5.2), by penetrating its cell membrane and depositing the male genetic material into the female cell, where the two nuclei fuse. The fertilised ovum (zygote) immediately becomes resistant to penetration by any other sperm arriving later. After fertilisation occurs, the zygote remains in the fallopian tube for about 72 hours, and during this time it develops rapidly, as you will see in the next section.