Period blood that we see every month is actually called nutrition bed for the fertilized egg to grow after it lays on somewhere inside the uterus. [1]
We are the reproductive beings and almost all of our hormones work to facilitate us every month to make us grow stronger and prepare in case there is birth happened. That is why we have period, and menstrual cycle is a reproductive natural system that makes life happened.
When hormone called (Endometrium) prepares a nutrition bed to feed the blastocyst, a rapidly dividing ball of cells. The inner group of cells will become embryo, the outer group will become the cells that nourish and protect it. The level of hormones (Estrogen) and (Progesterone) will increase to thicken the nutrition bed (they will tie together to become placenta later).[2]
Therefore, when a woman is pregnant, she will not have her period instead the blood will increase as layer after layer and become bed to provides oxygen, blood, and nutrition. [2] If egg explodes and do not meet the sperm, there will be no birth. Blood, discharge of the internal genital will all comes out as period blood. [3]
Special Part!
If you ever heard they said "Period blood is like washing all the internal bad stuffs from the uterus?" this is the theory that they dismiss ,but still discuss. The biologist Magie Frofet is the one comes up with that saying, she also mention that the morning sickness is the method of the body use to detox to protect mother and child.[4]
Though, if we come to think about the saying above, the fact is that only human has menstrual cycle to prepare for the guest (egg and sperm meet) that may never arrive. While some other mammals only need the period blood in the uterus when there is birth inside and some others do not have period. Is that mean others might doomed? without any period blood to wash away bad stuffs from their body?[4]
In the book "How Hormones Drive desire, shape relationships and make us wiser"
Hormones do its work every month in an efficient way as we believe that it wants to make sure the survival of life. While taking care of the new life, child in the uterus, as it needs aggressive nutrition at first ,as it merges itself so deeply into the Endometrium bed, it also need to make sure the mother is alive and well. [4]
With this information above, compare to the cycle of sperm that the way male body makes the ejaculate happens from time to time and that to make sure the new born sperms are the most healthy and especially to prepare for meeting with the egg in woman body by the heavy involvement of hormone Testorerone, of course. We can see that compare to menstrual cycle, our body really does its best in the repeated process and cycle to make sure of life survival indeed.
[1] Korin Miller "This is what actually in your periods bloods" July 15, 2016.;
[2] Tracee Cornforth, The Endometrium and Its Role in Reproductive Health, Periods, Pregnancy, and Potential Problems, Updated on April 13, 2023.
[3] Menstrual Cycle: An Overview, John Hopkins Medicine, https://www.hopkinsmedicine.org/health/wellness-and-prevention/menstrual-cycle-an-overview
[4] Martie Haselton, How Hormones Drive desire, shape relationships and make us wiser, p. 84-86, 2018.