If you’re a lady who has already hit puberty, your monthly menstruation cycle is by now a familiar visitor. But how much do you really know about what’s going on inside your body during period week? The process is a remarkably complex chain of events, and this TED video narrated by Emma Bryce illustrates it.

Menstruation cycles typically last between 2-7 days, and occur once every 28 days or so, although every woman is unique in her period timing. We know that the release of certain hormones — including follicle-stimulating hormone (FSH), luteinizing hormone (LH), estrogen, and progesterone — trigger the cycle. To set the stage, we also know that two ovaries are stocked with thousands of sacs called follicles, each containing one unfertilized egg cell, also known as oocytes. Ovaries contain thousands of egg cells, but they only release one each month. This results in either pregnancy or a period.

Each month, the pituitary gland, which is located in the brain, begins releasing LH and FSH into the blood. Once these hormones reach the ovaries, they trigger the egg cells to grow; during this time, the follicles release estrogen, which peaks at its highest at this point. The most mature egg cell is then released through the follicle, going through the ovary wall. This is called ovulation, and this occurs 2-16 days before a period even begins.

As the cell moves through the fallopian tube, it must be fertilized within 24 hours in order to start pregnancy — otherwise a period will occur. Now that the ovaries are empty, they’ll release progesterone, which causes the uterus lining to plump up with blood and nutrients. The entire reason the womb enlarges with blood is to prepare for a potential fertilized egg to land there and grow. If nothing arrives, the womb sheds blood and tissue, resulting in a period.

For more information about menstruation, read about how periods and changing hormone levels impact female’s bodies and cognitive function; the facts vs. fiction details about periods; and menstruation myths you should stop believing.