The menstrual period is when the unfertilised egg is expelled from the body. Since fertilisation has not taken place, the walls of the previously readied womb contract and the egg is expelled with the breaking of tiny blood vessels. Following that, the body will then begin preparations to repeat the whole process all over again.
All of these stages are repeated in all women over a specific period. Every month, new egg cells form, the same hormones are secreted-again and again at the same times. Thus the female body is prepared as if it will be fertilised. However, in the final stage, the absence or presence of the sperm changes the nature of the preparations in the body.
During the period in question, the changes in the empty space in the womb can only be identified by an anatomical or gynaecological examination. Yet these changes, only recently identified by scientists, are miraculously indicated in Surat ar-Ra'd:
Allah knows what every female bears and every shrinking of the womb and every swelling. Everything has its measure with Him. (Qur'an, 13:8)
At the beginning of the menstrual period, the mucous on the walls of the womb (the endometrium layer) is 0.5 mm thick. Under the effect of hormones secreted by the egg, this layer grows and reaches a thickness of 5-6 mm. This layer is then discarded in the absence of fertilisation. As we see from the above verse, this monthly increase and reduction in the walls of the womb is indicated in the Qur'an.