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Just about every one of your cells, tissues, and organs has some type of internal clock.

These timers sense light, dark, and other daily cycles, and they function in response to these rhythms. All of these biological clocks are coordinated by the primary synchronizer: your brain.

When people speak of biological clocks, however, they’re often talking about only one of these internal timers — the one related to human fertility. Why does this clock command so much attention? Is fertility really winding down like an analog clock?

In this article, we’ll help answer that question and take a closer look at the connection between your biological clock and fertility.

Fertility changes over the course of your lifetime. The term “biological clock” refers to the fact that it’s generally harder to get pregnant later in life.

There’s considerable scientific evidence that the number and quality of both eggs and sperm decline as you age.

But the idea of a biological clock has also come to symbolize the sense of psychological pressure you may feel when you haven’t had a child by a certain age.

In 1978, The Washington Post published an article by journalist Richard Cohen called “The Clock is Ticking for the Career Woman.”

It described a “composite woman” who had spent her most fertile years building her career instead of having babies. This composite woman sat at her desk, looked down at her midsection, and ached over her choices.

Though career opportunities for women have grown and infertility treatments have advanced since the 1970s, many people still feel pressure to have a child during their most fertile years.

This can be a source of intense anxiety, especially if preparing for your career has left you with little time to pursue parenthood or if people in your life frequently question you about your reproductive choices.

According to a 2011 research review, other reasons women may postpone having children may be due to:

  • inability to find a suitable partner
  • housing and economic uncertainty
  • the rise of cohabitation before marriage
  • increased divorce rates
  • a shift in norms and values

Just as women start and stop menstruating at different ages, the age of peak fertility also differs from person to person.

Research shows that ideas about fertility also vary.

In one 2018 study involving 990 U.S. citizens, most people said age 23 was the ideal age for a first pregnancy and by age 49 a woman would probably be infertile. In that study, education, race, age, and income all influenced beliefs about the ideal age to have a baby.

Although scientists don’t all agree on the specific age at which women are most fertile, the consensus is that fertility peaks in the early 20s and begins to decline after age 32, according to the American College of Obstetricians and Gynecologists (ACOG).

After age 37, many women have a much harder time becoming pregnant. For many men, fertility begins to decline in their 40s.

When a female baby is in the womb, they can contain as many as 7 million eggs (also called oocytes) in their ovaries, according to ACOG. From that point on, the number of eggs drops steadily and naturally over time.

By the age most females reach puberty, between 300,000 and 500,000 eggs remain. When menopause arrives, which is usually around age 51, the ovaries may contain around 1,000 eggs.

The health of your eggs and the level of reproductive hormones in your body also decrease over time. At the same time, your risk for certain conditions goes up as you get older.

Some examples of conditions that may make it harder to get pregnant include:

If you have a history of any of these conditions, you might want to talk with your healthcare provider about possible effects on your fertility.

Statistics published by the Centers for Disease Control and Prevention (CDC) show that around 12 percent of people between ages 15 and 44 don’t get pregnant after they’ve tried for a year or don’t carry the pregnancy to term.

The main symptom of infertility is not getting pregnant if you’re having sex without using birth control.

There may be other indications that getting pregnant may be harder. For example, if you have irregular periods, your body might not be releasing eggs on a regular schedule, either.

If your periods are more than 35 days apart or if they’re too close together (21 days or fewer), ovulation might be disrupted, too.

For men, symptoms of infertility might include:

If you’re not ready to get pregnant now, you may be able to remove some of your eggs and preserve them for a time when you’re ready to be a parent.

Mature oocyte cryopreservation is a procedure that allows a healthcare provider to harvest several of your eggs, freeze them, and thaw them later.

When you’re ready to get pregnant, the eggs can be fertilized and implanted through a process call in vitro fertilization (IVF). It’s also possible to freeze fertilized embryos.

A newer procedure that involves the freezing of ovarian tissue may also be an option. However, it’s important to note that the success rate of this procedure is lower than that of egg freezing.

Egg freezing carries some risks. That’s why you should discuss your options with a physician before you decide whether it’s the right solution for you.

What to know about the risks of freezing your eggs

Although roughly 84 percent of cryopreserved eggs survive, there are no guarantees that egg freezing and IVF will lead to a healthy pregnancy later. Also, the cost of egg freezing can make this a difficult option for some people.

Before your eggs can be harvested, you’ll probably be placed on hormones that cause your body to release several eggs at once.

These hormones can cause a painful reaction called ovarian hyperstimulation syndrome. Most of the time, this syndrome goes away as soon as the hormones stop. However, there’s a very small chance that it could become life threatening

Although you’ll be sedated when your eggs are removed, the process of harvesting eggs involves the use of a needle.

It’s not uncommon to experience cramping or bleeding afterward. In rare cases, injuries and infections have occurred.

Becoming pregnant later in life carries some risks, including a greater risk of:

The biological clock is a metaphor used to describe the sense of pressure many people feel to get pregnant while they’re at the peak of their reproductive years.

While it’s true that fertility begins to decline for most people in their mid-30s, you can still become pregnant later in life.

The possibility of harvesting and freezing eggs for fertilization later in life is becoming more socially acceptable. But the procedure has risks that should be carefully considered before choosing this means of delaying pregnancy.

If you’re feeling conflicted about when or whether to have a child, you’re definitely not alone. It might be a good idea to talk with people you trust — including a healthcare provider — about what decision is best for your physical and mental health as well as that of your future child.

Your reproductive choices are a deeply personal matter, and you have the right to make them in your own time.