Many females are born with all the immature egg follicles they’ll ever have — about 1 to 2 million. Only about 400,000 of those eggs remain at the start of menstruation, which occurs around age 12.

With each period, several hundred eggs are lost. Only the healthiest follicles will become mature eggs. The body breaks down and absorbs the rest. Males, on the other hand, continue to create new sperm for most of their adult lives.

As the body ages, it has fewer follicles. That means the follicles have fewer opportunities to create healthy, strong eggs for fertilization. In the teenage years, the supply is robust, but by the late 30s and 40s, the supply is waning. However, that’s the age when more and more people are attempting to start a family.

Today, the average age of giving birth for the first time is 26.6 years old. That age has been steadily increasing in recent years as parenthood is postpone.

Let’s take a look at how the decision to wait can affect your fertility.

If ever there was a “best” age to procreate purely from a physical standpoint, this would be it.

Your body’s strongest ovarian follicles are the first to mature into eggs for ovulation, so the eggs you produce in your younger years are more likely to be high quality.

Having a child at this age will cut the risk for:

  • birth defects
  • chromosomal problems
  • some fertility issues

Of course, while it’s less risky to have children when you’re 18 to 24, it’s not without risk.

This fertility chance, also known as fecundity rate, will wax and wane through your life. It’s at its strongest in this younger age period. Between ages 20 and 30, the natural fertility rate each month is about 25 percent. That dips to below 10 percent after age 35.

Birth rates are dropping for females ages 18 to 24. Many are foregoing families for careers until they’re in their 30s and 40s.

With each passing year, your chance of naturally conceiving a child falls. But in your late 20s, your chance of getting pregnant without intervention remains fairly steady.

In fact, couples under age 30 who are otherwise healthy are able to conceive in their first three months of trying 40 to 60 percent of the time, estimates the Eunice Kennedy Shriver National Institute of Child Health and Human Development. After age 30, the chances of getting pregnant begin to decrease every year.

If you haven’t started a family yet, don’t worry! Your body still has a generous supply of eggs to provide when the time is right.

But if you’re trying to conceive and have been unsuccessful for at least three months, talk with your doctor. While most couples at this age will be able to have a baby without intervention, some guidance may be helpful.

In your early 30s, the chances you’ll be able to have a baby are still high.

You still have a lot of high-quality eggs to offer, but your odds will start to decline steadily at this age. Your fecundity rate decreases gradually until age 32. At 37, it drops dramatically. In your 30s, you’re about half as fertile as you are in your early 20s.

Does that mean you can’t have children if you’re in your 30s? Absolutely not.

In fact, 1 in 5 females nationwide have their first child after age 35, notes the National Institutes of Health. However, 1 in 3 couples in their 30s will experience some type of infertility issue.

The greatest reduction in fertility is in the late 30s and early 40s. The chances a female in their late 30s will be able to conceive spontaneously is about half that of a female in their early 20s.

A 2003 review notes 60 percent of couples in this age range will be able to conceive naturally within a year of starting to try, while 85 percent will be able to conceive within two years.

However, at this age, the risks of chromosomal issues with eggs are higher. The risks increase with every additional year. That means the risks of miscarriage or abnormal pregnancy are higher.

This fall in fertility rates happens to coincide with the decade of life when more people than ever are trying to get pregnant.

From 2011 to 2016, the birth rate for females ages 35 to 39 rose each year, falling 1 percent in 2017, reports the Centers for Disease Control and Prevention (CDC). For females over 39, the birth rate is even higher.

According to the CDC, birth rates for ages 40 to 44 increased 2 percent between 2016 and 2017. The number of births for females 45 to 49 rose 3 percent in the same time frame. In fact, the fastest-growing rates of childbearing are in females 40 and older.

It’s important to keep in mind, however, that while more individuals are giving birth at these ages, the overall percentage of births to older parents is still much lower than in younger ones. That’s due, in part, because it’s tougher to get pregnant if you’re over 40.

By this age, your body is preparing for menopause. Your ovaries have likely exhausted their follicles or are nearing the end of their supply. With each passing cycle, more will disappear. By the time you reach your early 50s, you’ll have almost no follicles remaining.

Babies born from people in this age range are also at greater risk for a number of birth defects and pregnancy complications. Miscarriage and chromosomal abnormalities increase significantly during this period of life.

Older age also increases the risk of complications for the parent, including:

  • diabetes
  • hypertension
  • preeclampsia

Today, people are waiting longer to start families. Because of advancements in fertility treatments, such as in vitro fertilization, these individuals often do succeed at getting pregnant at this later stage.

While your natural window gradually closes with your age, fertility treatments may be able to extend your window and even make your chances of successful conception higher.

Kimberly Holland is a health and lifestyle writer and editor based in Birmingham, Alabama. When not organizing her books by color, Holland enjoys traveling, toying with new kitchen gadgets, and exploring small-town restaurants and stores.