Why does nature give women a “fertility deadline”… but not men?

A question comes up frequently: why does a woman’s fertility decrease with age, while a man may still be able to father a child at an advanced age? Is this a biological injustice? The reality is much more complex.

Two completely different biological systems

The main difference lies in the way reproductive cells are produced.

In women, oocytes are formed before birth. A baby girl is born with a limited number of oocytes stored in her ovaries. Throughout her life, she will use only a small proportion of them: during each menstrual cycle, several follicles begin to develop, but usually only one oocyte reaches maturity and can potentially be ovulated.

A woman therefore does not produce new oocytes after birth. Over time, this ovarian reserve naturally decreases. This is known as the ovarian reserve.

However, fertility does not depend only on the number of available oocytes.

Quantity… but above all, oocyte quality

With age, the ovarian reserve gradually decreases, but one essential aspect is often overlooked: oocyte quality also changes over time.

Oocytes have been present in the ovaries since fetal life. Over the years, they age as well. With increasing maternal age, the risk of chromosomal abnormalities in oocytes rises, which explains:

• a decrease in the chances of conception;

• an increased risk of early miscarriages;

• an increased risk of chromosomal abnormalities in embryos.

Therefore, a woman may still have oocytes available, but their reproductive potential may differ from that of a younger woman.

This is why female age is one of the most important factors influencing fertility, regardless of regular menstrual cycles or the presence of ovulation.

In men: continuous production, but not eternal youth

In men, the system works differently. Sperm cells are continuously produced from stem cells located in the testes.

The production of a new generation of sperm takes approximately 70 to 90 days. This is why it may appear that men could remain fertile throughout their entire life.

But this impression is misleading.

Male fertility also changes with age. Over time, we may observe:

• a possible decrease in sperm concentration and motility;

• an increase in sperm DNA fragmentation;

• an increase in certain new mutations that may be transmitted to the child.

Men therefore do not have a clearly visible “expiration date” like women, but they also have a biological fertility window.

Why are the effects of lifestyle not exactly the same?

These differences in reproductive cell biology also explain why certain life events do not have the same impact over time.

In men, sperm cells are constantly renewed. A period that affects sperm production can therefore influence sperm quality in the following months.

High fever, certain toxic exposures, some medications, severe stress, or excessive alcohol consumption can affect sperm quality for approximately three months — the time needed to produce a new generation of sperm.

In women, oocytes have existed for decades. A short period of excessive alcohol intake or a difficult few weeks does not mean that ovarian reserve suddenly disappears.

This does not mean that lifestyle has no importance: smoking, certain diseases, or certain treatments can have a significant impact on female fertility. However, the biological mechanism is different.

What is the real fertility window?

Female fertility is highest in younger women and then gradually decreases with age. This decline becomes more noticeable after 35 and accelerates further after 40.

The reason is not only the decrease in the number of oocytes, but also the evolution of their quality.

In men, fertility can remain possible for much longer, but advanced paternal age is not without consequences. From around 40–45 years of age, sperm parameters may gradually change and certain risks may increase.

So, is nature unfair?

From a human perspective, it may seem unfair: a woman who wishes to become pregnant at 45 does not have the same biological possibilities as a woman aged 25, while a man of the same age may still conceive a child.

But biologically, this is not a preference of nature for one sex over the other. The two reproductive systems have simply evolved differently.

A woman carries a pregnancy for several months, requiring major energy expenditure and profound adaptations of her body. Evolution has favored a limited but carefully regulated stock of oocytes rather than continuous renewal.

A man produces millions of sperm cells throughout his life, but this continuous production also comes with a progressive risk of errors linked to repeated cell divisions and aging.

Two different biological clocks

The reality is therefore more nuanced:

A woman has a more visible biological clock, linked to the decline of ovarian reserve and changes in oocyte quality.

A man is not outside the effects of time: his sperm cells are renewed, but their quality also evolves with age.

Fertility is a window in both sexes. Nature simply did not set these two biological clocks in the same way.

Take care of yourself!

Dr. Sandra Yene Amougui