Fertilisation and Implantation: Fertilisation and Implantation are the two major processes in reproduction in human beings. The zygotes fuse and are later implanted on the walls of the uterus for further growth. During fertilization, the sperm and egg unite in one of the fallopian tubes to form a zygote. Then the zygote travels down the fallopian tube, where it becomes a morula. Once it reaches the uterus, the morula becomes a blastocyst. The blastocyst then burrows into the uterine lining a process called implantation.
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.
Implantation: Between five to seven days after fertilisation, the blastocyst reaches the uterus and embeds itself in the thickened endometrium (lining of the uterus). This process is called implantation, and if the embryo survives it is the beginning of a pregnancy. However, the embryo may not implant, or it may not survive for more than a few days. In this case, it is shed from the uterus as the endometrium breaks down, and it passes out of the vagina in the menstrual fluid. The loss of a very early pregnancy in this way is very common, and the woman does not even know that she was momentarily pregnant.
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