Fertilisation is the process in which the males sperm enters the females ova. In order to fertilise the egg, the egg has to be clean so that the sperm enters freely. Usually, the females egg gets accumulated by filthy matter and waste. This blocks the sperm from entering. So, just a week before the fertilisation, menstruation occurs in which the blood washes the egg and removes the dust. That is why, after a week of menstrual cycle, fertilisation occurs.
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