Many of us are pretty in tune with our bodies. You can likely immediately point to that tight spot on your right shoulder that knots up when you’re tense. Yet, you may like to know a whole lot more about what’s going on inside your body, such as, “What’s the story behind my eggs?”

Yes, female babies are born with all the egg cells they’re ever going to have. No new egg cells are made during your lifetime.

FYI: Egg terminology

An immature egg is called an oocyte. Oocytes rest in follicles (fluid-filled sacs that contain an immature egg) in your ovaries until they begin to mature.

The oocyte grows up to be an ootid and develops into an ovum (plural: ova), or mature egg. Since this isn’t a science course, we’ll mainly stick to the word we’re most familiar with — egg.

As a fetus early in development, a female has around a whopping 6 million eggs. The number of these eggs (oocytes, to be precise) is steadily reduced so that when a baby girl is born, she has between 1 and 2 million eggs. (Sources differ slightly, but regardless, we’re talking about a seven-digit figure!)

Good question. The eggs are there, so what’s stopping the menstrual cycle from starting up?

The menstrual cycle is on hold until a girl reaches puberty. Puberty begins when the hypothalamus in the brain starts to produce gonadotropin-releasing hormone (GnRH). In turn, GnRH stimulates the pituitary gland to produce follicle-stimulating hormone (FSH). FSH initiates egg development and causes estrogen levels to rise.

With all of this going on inside of us, it’s no wonder some of us experience the associated mood swings!

Wondering about the first sign of puberty? Menstruation starts about 2 years after the breast bud — that little bit of tender tissue that develops into a breast — appears. While most girls begin to menstrate around age 12, others can start as early as 8.

When a girl reaches puberty, she has between 300,000 and 400,000 eggs. Hey, what happened to the rest of those eggs? Here’s the answer: Before puberty, about 11,000 die each month.

The good news is that the number of eggs that die each month decreases after puberty. After starting her menstrual cycle, a woman loses about 1,000 (immature) eggs every month, according to Dr. Sherman Silber, who authored “Beating Your Biological Clock,” a guide for his infertility clinic patients. That’s about 30–35 per day.

Scientists aren’t sure what prompts this to happen, but they know that it isn’t influenced by anything we can control. It’s not influenced by your hormones, birth control pills, pregnancies, nutritional supplements, health, or even your intake of chocolate.

Once follicles mature, they finally become sensitive to the hormones of your monthly menstrual cycle. However, they aren’t all winners. Only a single egg ovulates. (Usually, at least. There are exceptions, which in some cases lead to fraternal twins.)

Given the numbers, when a woman reaches 30, her fertility begins to decrease. By the time she reaches 40, if she’s like most of us, she’ll be down to about 3 percent of her pre-birth egg supply.

Related: What to know in your 20s, 30s, and 40s about getting pregnant

So you’ve hit 40. There’s no one-size-fits-all answer to how many eggs you have left. What’s more, certain factors — like smoking — may mean you have fewer than another woman.

Research has shown that the average fertile woman becomes relatively infertile by 40 (with less than a 5 percent chance of getting pregnant per cycle) and undergoes menopause by 50. Crunch the numbers and you see that when only 25,000 eggs are left in the ovaries (around age 37), you have about 13 years until you reach menopause, on average. Some will hit menopause earlier, and some will hit it later.

Related: What you should know about having a baby at 40

We’ve talked a lot about the quantity of eggs you have. But what about the quality?

Just before ovulation each month, your eggs begin to divide. Older eggs are more prone to errors during this division process, making it more likely that they’ll contain abnormal chromosomes. This is why the chances of having a baby with Down syndrome and other developmental abnormalities increase as you age.

You can think of your egg reserve as a little army. The strongest soldiers are on the front lines. As the years go by, your eggs are ovulated or discarded, and older, lower quality ones remain.

When you run out of your supply of viable eggs, your ovaries will cease to make estrogen, and you’ll go through menopause. Exactly when this happens depends on the number of eggs that you were born with.

Remember that discrepancy between 1 or 2 million? If you were born with a larger number of eggs, you may be among the women who are able to have biological children naturally into their mid- or even late 40s.

Related: Having a baby at 50

Are you having trouble getting pregnant — or are you approaching 40 or older and starting to think about pregnancy? Now that you have the numbers, you’ll be better equipped to discuss your options with your OB, who can also run tests on your egg reserve and egg quality.

One route you may consider is oocyte vitrification. If your OB offers you oocyte vitrification or elective fertility preservation (EFP), know that they’re talking about freezing your eggs. Many women who consider EFP are motivated by the ticking of their biological clock. Others may be about to start chemotherapy treatments that could affect their fertility.

Considering EFP? According to one source, your chances of having a child are better when you do EFP when you’re younger than 35.

Other reproductive technologies, such as in vitro fertilization, are also allowing women in their 40s — and even 50s — to achieve pregnancy. Talk to your doctor about your unique situation and know that you have options.