Question: why do girls have monthly flow of blood

Makobu Kimani answered on 17 May 2018:

The uterus is made up of layers of different tissues. The innermost layer is called the endometrium. This is where a foetus would be attached to in the event that conception happens. However the endometrium is not always ready for attachment of a foetus. It is determined by hormones released by both the ovaries and the pituitary gland. When all the conditions are right, the ovaries release an egg (ovum) into the fallopian tubes. Here it awaits fertilization by a sperm. At the same time, the endometrium thickens and developed new capillaries in preparation for attachment of a foetus.
If Fertilization does not occur within a set time, the ovum, under influence of changing hormones, degrades and as it travels toward the endometrium, causes the thickened layer also breaks off and as it does all this turns into what we now see at menstrual blood.