[header] How to tell your mom about your period [title] Learn what having your period means. [step] The menstrual cycle is a monthly process your body undergoes in preparation for pregnancy. In the beginning of the cycle, your body creates more estrogen, which causes the lining of the uterus to thicken with blood and mucous. 
At the same time, the ovaries release an egg. An egg that gets fertilized by male sperm would attach itself to the thickened uterine wall.