Fertility 101: How Your Body’s Reproductive Clock Works

If you are thinking about starting a family, freezing your eggs, or you just want to understand how your fertility actually works, we will talk through it in simple terms in this article. What your body’s timeline looks like, and what that elusive “biological clock” is really ticking toward.

First, What Is the “Biological Clock”?

You’ve probably heard that phrase thrown around, “Your clock is ticking,” they say. But what does it actually mean?

The biological clock is just a way of referring to how fertility changes with age. For women, this generally revolves around egg quantity and quality. You're born with all the eggs you'll ever have, about 1 to 2 million. By puberty, that number drops to around 300,000. And each month, your body recruits a small group of eggs, and typically just one is released through ovulation.

Over time, the remaining eggs decrease both in number and in quality. That means they’re more likely to carry chromosomal issues or simply not develop properly. This is why fertility tends to decline as women reach their mid-30s and beyond.

Your Hormones

Fertility is influenced by a set of hormones that work together throughout your cycle. Each one plays a specific role in preparing your body for the possibility of pregnancy.

FSH (Follicle-Stimulating Hormone): Supports the development of eggs in your ovaries.
LH (Luteinizing Hormone): Triggers ovulation, which is when the egg is released.
Estrogen: Helps thicken the uterine lining and supports the early part of the cycle.
Progesterone: Maintains the uterine lining after ovulation to support implantation.

These hormone levels rise and fall at different times during your menstrual cycle. If there’s an imbalance, such as with conditions like PCOS or thyroid disorders, it can disrupt the process and affect fertility.

Understanding Your Fertility Timeline (By Age)

While fertility varies from person to person, there are some general patterns that tend to follow age. These are average ranges, not fixed rules.

In your 20s:

Fertility is typically highest during this time.
The chances of conceiving each cycle are around 25 to 30 percent.
Egg quality is generally good, and the risk of miscarriage is lower.

In your 30s:

Fertility may start to decline gradually around age 32.
After 35, the decline often becomes more noticeable.
The chances of miscarriage and chromosomal conditions increase with age.

In your 40s:

Fertility declines more significantly after 40.
By 45, natural conception is less likely for many people.
Fertility treatments like IVF may still be an option, though success rates tend to be lower.

What Affects Fertility (Besides Age)

Age is one factor, but several other elements can also influence fertility:

Lifestyle: Smoking, heavy alcohol use, poor nutrition, and low physical activity can all reduce fertility.
Weight: Being significantly underweight or overweight can interfere with ovulation.
Medical conditions: Issues like PCOS, endometriosis, fibroids, and thyroid imbalances can affect the reproductive system.
Stress: Long-term stress may alter hormone levels and affect menstrual regularity.
Environmental exposures: Contact with certain chemicals, like those found in some plastics, may impact hormone balance.

Men’s fertility also changes with age. While the shift is more gradual, sperm quality and DNA integrity can decline over time, especially after age 40.

Tracking Fertility

There are several ways to monitor your fertility, especially if you’re trying to conceive or simply want to understand your cycle better.

Ovulation Predictor Kits (OPKs): These test for hormone changes that indicate when ovulation is approaching.
Basal Body Temperature (BBT): Tracking your temperature each morning can help identify when ovulation has occurred.
Cervical mucus monitoring: Fertile cervical mucus is typically clear and stretchy. Observing these changes can help pinpoint your most fertile days.
Cycle-tracking apps: Digital tools can help monitor your cycle, but it’s best to use them alongside physical signs for accuracy.

If you’ve been trying to get pregnant for a year without success, or for six months if you’re 35 or older, it may be time to speak with a fertility specialist.

Understanding fertility is about more than planning for pregnancy. It’s about having accurate information about how your body works so you can make informed choices, whether that means starting a family now, later, or not at all.

Everyone’s path is different. Knowing your body and being aware of your options can help you feel more in control, whatever your goals may be.