When the ovaries develop in utero, there are about 6 million eggs in them. By the time the baby is born, there are 1-2 million. Before puberty, a girl loses about 10,000 eggs per month. After puberty, her hormones have a protective effect and she only loses about 1,000 a month.

After puberty, only one egg matures and drops per month; the rest just resorb.

This explains it in more detail: https://www.healthline.com/health/womens-health/how-many-eggs-does-a-woman-have#eggs-lost-each-month

During every cycle, the best egg at the time is released that has the highest chance of fertilization. This means there's competition every month which increases reproductive chances. If there were only 600 eggs then there wouldn't be competition and if a bunch of eggs were damaged for any reason then this would decrease reproductive chances and introduce unintended mutations.

So cramps are actually monthly tournaments with egg on egg combat to determine who's the strongest?

No but that would be such a neat idea :]

Cramps is what happens when cells that line the uterus (endometrial cells) break down. This processes releases chemicals (prostaglandins) that cause the muscles and blood vessels in the uterus to contract.

Because we evolved from animals that released millions of eggs in their lifetime and thousands at once. There is no evolutionary pressure for you to produce fewer eggs even though we evolved to produce very few offspring.

Also for most of human evolution, life expectancy was much less than 50 years, so there was no evolutionary pressure to be fertile for longer.

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