Answer :
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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because when mensuration cycle take place in womens at reaching the age of puberty one egg is produce every month and thick layer of blood is surrounded around utereus for nourishment of embryo but if their is unfertilised egg it lead to bleeding it is known as mensturation cycle . we say that bleeding through uterues is a signal that there is presence of ova in oviduct so fertilisation can take place.
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