Many of us are pretty in tune with our bodies. For example, you can likely immediately point to that tight spot on your right shoulder that knots up when you’re tense.

Yet, you might like to know a whole lot more about what’s going on inside your body. Maybe you’re asking, “What’s the story behind my eggs?”

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

This has long been accepted as fact.

Yet, reproductive biologist Jonathan Tilly offered research in 2004 that initially claimed to show new egg stem cells in mice. This theory has been generally refuted by the wider scientific community, though there is a small group of researchers still pursuing this work. (A 2020 article in The Scientist describes the debate.)

FYI: Egg terminology

An immature egg is called an oocyte. Oocytes rest in follicles (fluid-filled sacs) 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 name we’re most familiar with — egg.

As a fetus early in development, a baby with ovaries has around a whopping 6 million eggs.

The number of these eggs (oocytes, to be precise) is steadily reduced so that when the baby is born, they have between 1 and 2 million eggs left.

Sources differ slightly on this number, 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 you reach puberty. Puberty begins when the hypothalamus in your brain starts to produce gonadotropin-releasing hormone (GnRH).

In turn, GnRH stimulates the pituitary gland to produce follicle-stimulating hormone (FSH). FSH begins the process of egg development and causes estrogen levels to rise.

During puberty, menstruation starts about 2 years after the breast bud — that little bit of tissue that develops into a breast — appears. While the average age is 12, others can start as early as 8 years old, and most will start by age 15.

When you reach puberty, you have between 300,000 and 400,000 eggs. So, what happened to the rest of them? Here’s the answer: Before puberty, more than 10,000 eggs die each month.

Once follicles mature, they finally become sensitive to the hormones of your monthly menstrual cycle.

Every month, the body selects one egg out of the group to become the dominant follicle. This follicle contains the one egg that is ovulated that month and represents your one chance to conceive. (Usually, at least. There are exceptions, which in some cases lead to fraternal twins.)

All the others eggs present in that cycle have not been selected as the dominant follicle, so they undergo atresia, which means that they die. This happens every month: You ovulate one egg and the rest die, and that cycle continues until menopause when there are no eggs left.

The number of eggs that die each month decreases after puberty, though.

After starting the menstrual cycle, a person loses about 1,000 immature eggs every month, according to Dr. Sherman Silber, who authored “Beating Your Biological Clock,” a guide for his patients who have trouble with fertility.

Scientists aren’t sure what prompts this to happen, but they know that it is not influenced by most things we can control. For example, it is not influenced by:

  • your hormones
  • birth control pills
  • pregnancies
  • nutritional supplements
  • health
  • food choices

Some exceptions: Smoking accelerates egg loss, according to 2018 research. Certain chemotherapies and radiation do, too.

When you reach age 32, fertility begins to decrease. It declines more rapidly after 37, according to the . By the time you reach 40, if you’re like most of us, you’ll be down to less than 10 percent of your 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 person.

Research has shown that the average woman has less than a 5 percent chance of getting pregnant per menstrual cycle.

Plus, the average age of menopause in the United States is 51. Some will reach menopause earlier and some will reach it later.

Crunch those numbers and you see that when only 25,000 eggs are left in the ovaries (around age 37), you have about 15 years until you reach menopause, on average. At menopause, you have fewer than 100 eggs left. Some people will approach menopause earlier, and some will reach it later.

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

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

The most important factor that determines egg quality is age. As you age, both the quantity and the quality of your eggs decreases.

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, according to ACOG.

As egg quality decreases, the genetic material becomes more damaged and less reparable. So, when the egg and sperm combine to form an embryo, there is a higher chance of chromosomal abnormalities or genetic defects.

This is why the chances of having a baby with Down syndrome and other chromosomal or genetic differences increase as you age. A chromosomal abnormality is the most common cause of a pregnancy ending before term.

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 and how quickly you lose your eggs.

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.

On the other hand, there may be women in their 30s who experience a faster rate of egg loss due to certain risk factors. As a result, they may have fertility concerns or may even experience early menopause or ovarian failure.

Related: Having a baby at 50

Are you having trouble getting pregnant? Now that you have the numbers, you’ll be better equipped to discuss this with a healthcare professional.

If you have questions or concerns about your ovarian reserve or your egg quality, be sure to speak with a board certified fertility doctor who can help assess your risk factors and discuss your options.

If you’re concerned that time is not on your side, you may want to consider freezing your eggs, aka oocyte vitrification or elective fertility preservation (EFP).

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. (Note: Egg freezing before chemo isn’t considered “elective” because it is medically indicated fertility preservation.)

Considering EFP? According to one source, your chances of having a child with your frozen eggs are better if you freeze before you’re age 35.

It’s also important to remember that the more eggs you freeze, the better your chances will be of having a baby if you choose or need to use those eggs. You are more likely to freeze a higher number of better quality eggs if you are younger.

Reproductive technologies, such as in vitro fertilization, are allowing people to start or grow their families much later in life.

Please note that IVF with your own eggs is unlikely to be a viable option for a person with fertility trouble who is past the early 40s, according to research from 2014. However, donor eggs from a younger person can allow people in their 40s and 50s to conceive.

Talk with your doctor early and often about fertility plans and how fertility can change over time. Know that you have options.