We mention this time and time again here on our blog: age plays a huge, important role in your fertility. We’ve discussed the biological clock and the scientific realities associated with this natural mechanism found in all of our bodies. We’ve explored the evidence a woman’s fertility does naturally decline with age, more so than a man’s does. But the question is: why?

Women are born with all the eggs they’ll ever have.

Before birth, we have 6 to 7 million potential eggs in our ovaries. When we’re born, that number drops to around 1 to 2 million, and by the time we reach puberty, only about half of those eggs remain. Then, during our reproductive years, we lose more eggs during each menstrual cycle—hundreds or even thousands more.

Here’s the nitty-gritty: the average cycle takes 28 days and occurs in three phases. First, there’s the follicular phase (when the egg is developed); then, there’s the ovulatory phase (when the egg is released); and finally, there’s the luteal phase (post-ovulation, when the body is prepping for a potential pregnancy). During the follicular phase, hormones released from our brains stimulate the growth of about 15–20 egg follicles in the ovaries. Of these, one will become dominant and continue on to become mature and be released; the other follicles will stop growing and die. (The death of ovarian follicles that don’t make it to maturity is called “atresia.”)

Plus, in addition to the 15–20 “activated” follicles that die each month, doctors estimate we lose up to a thousand additional potential eggs. Each cycle. And the number that we lose each month increases as we age. Contrary to popular belief, it’s not just the mature egg we lose every month that depletes our “egg bank,” but also the hundreds of other eggs that don’t reach the mature stage.

By the time a woman is 30, it’s estimated that she has only about 12% of her original quantity of eggs (the ones she had at birth), and only 3% by age 40.

As we age, our eggs age, too.

Our eggs are with us from before birth—and we never make any new ones. As eggs age, they are more likely to contain chromosomal abnormalities created during the division process (meiosis)—little mistakes in the new copy of their DNA. Because the DNA is essentially the “instruction booklet” for a cell, chromosomal abnormalities can result in the cell not doing what it’s supposed to do—in the case of the egg, making a healthy baby.

Egg quality is not a spectrum. An individual egg is either genetically normal (euploid) or genetically abnormal (aneuploid). An abnormal egg usually won’t fertilize. If it does, it has a higher likelihood of resulting in an early miscarriage or, in rare cases, genetic disorders such as Down Syndrome. (That’s why we see an increase in infertility, miscarriage, and genetic disorders in women over 35.)

Typically, 80–90% of a 20-year-old woman’s eggs are genetically normal, but—surprise, surprise—that percentage drops as the woman gets older. No 40-year-old is blessed with only normal eggs, because DNA damage is inevitable over time. It’s estimated that, by age 44, less than 20% of a woman’s eggs are genetically normal.

But other mammals can reproduce until their 80s! Why not humans?

While it’s true that other mammals such as whales and elephants have much longer reproductive lives than humans, their lifespans don’t extend much beyond their post-reproductive years. Apes, for instance, have a reproductive capability that begins to drastically decline in their late 30s—like us. But they don’t live much longer than those years. Humans, on the other hand, can live healthily for many years after menopause. What’s the deal?

Researchers aren’t exactly sure, but there seem to be two possible explanations, both based in evolutionary biology. The “mother hypothesis” argues that, for older mothers, it’s evolutionarily advantageous to invest resources in and focus on existing children than to continue to have more. The “grandmother hypothesis” proposes that women live past their reproductive years so that they can help provide and care for their grandchildren. That’s why their reproductive capacity drops just as the next generation’s is at its peak.

The good news for modern women trying to plan their futures? Even though our biology hasn’t quite caught up, the restrictions placed on us by our evolutionary history—a time when humans were lucky to make it to 40—don’t really apply today. Medical advances mean that not only can we live long enough to get wrinkles, we also have options for how we want to live our lives, and when—or if—we want to have kids. Cheers to that.