How Age Affects Fertility in Both Men and Women — What the Research Actually Shows

Age and fertility is one of the most discussed topics in reproductive medicine — and one of the most asymmetrically discussed. Almost all of the public conversation focuses on women. The decline in female fertility with age is well-documented, frequently cited, and a source of significant anxiety for many women.

What is far less discussed is the parallel reality for men: that male fertility also changes meaningfully with age, in ways that affect conception time, pregnancy outcomes, and the children born from those pregnancies.

Here is the full picture, for both partners.

How Age Affects Female Fertility

Egg Quantity

Women are born with approximately one to two million eggs. By the time of the first menstrual period, roughly 300,000–500,000 remain. Each cycle, a cohort of follicles is recruited; most undergo atresia (natural cell death) rather than ovulating. Only one egg typically ovulates per cycle.

This process continues whether or not a woman is pregnant or on hormonal contraception — the pool is always reducing. The pace of reduction is gradual through the 20s and early 30s, and accelerates from the mid-30s.

AMH levels — the marker most commonly used to estimate ovarian reserve — decline across the reproductive lifespan. But importantly, this decline is highly individual. Two women at 37 can have very different AMH levels. Population averages describe trends; they do not predict individual situations.

Egg Quality

After quantity, egg quality is the more critical variable with age. As women age, the cellular machinery that ensures correct chromosome separation during egg maturation becomes less precise. This leads to higher rates of chromosomal errors (aneuploidy) in the eggs of older women.

Aneuploid eggs either do not fertilise, do not result in implantation, or result in early miscarriage. This is why miscarriage rates increase with maternal age — from approximately 10–15% in the 20s to 30–40% by the late 30s and higher still into the 40s.

Conception Time and Treatment Response

Even when conception does occur, it often takes longer in older women — reflecting the lower per-cycle probability. IVF success rates decline with age: approximately 35–40% live birth per cycle under 35, declining progressively through the late 30s and 40s.

How Age Affects Male Fertility

This is the part most couples do not know — and that most doctors do not discuss unless directly asked.

Sperm DNA Fragmentation

The most significant age-related change in male fertility is the increase in sperm DNA fragmentation — the proportion of sperm with damaged genetic material. Unlike the relatively sudden shift in female fertility around the mid-30s, DNA fragmentation increases gradually but meaningfully from the mid-30s in men.

High sperm DNA fragmentation is associated with:

• Longer time to conception
• Higher rates of miscarriage (particularly early miscarriage)
• Lower fertilisation rates in IVF
• Possible effects on embryo quality and long-term child development (though research is ongoing)

This is not measured in a standard semen analysis. Standard semen analysis measures count, motility, and morphology — not DNA integrity. Sperm DNA fragmentation testing is a separate, additional test.

Standard Semen Parameters and Age

The relationship between standard semen parameters (count, motility, morphology) and age is less clear-cut than the relationship with DNA fragmentation. Some studies show modest declines in standard parameters with age; others show no significant change. The consensus is that DNA fragmentation is the more clinically meaningful age-related change for men.

Paternal Age and Pregnancy Outcomes

Research on paternal age has grown significantly in the last decade. Key findings:

• Men over 40 have meaningfully higher rates of sperm DNA fragmentation than men under 30, even when standard semen analysis results are normal.
• Pregnancies with a male partner over 45 are associated with higher rates of miscarriage, lower IVF success rates, and increased risk of certain developmental conditions in offspring — though absolute risks remain small.
• Time to conception increases with paternal age, particularly above 40.

A landmark study found that pregnancies with fathers over 45 were associated with higher risks of several conditions in children, including autism spectrum disorder and certain psychiatric conditions — though causality is complex and absolute risk remains low.

What This Means for Both of You

The practical implications of this research are straightforward:

For women: Your individual situation — your AMH, your cycle regularity, your overall health — matters more than your age in isolation. The population statistics set context; your specific test results set your situation.

For men: Standard semen analysis is not a complete picture of male fertility at older ages. If you are over 35 and your partner is struggling to conceive, sperm DNA fragmentation testing is worth discussing with your doctor.

For couples: Age is a reason to act sooner rather than later — not because things are hopeless, but because earlier information means more time to act on what is found. The gap between "everything feels fine" and "something has been affecting our chances" often exists precisely because neither partner has been tested.

The Right Next Step

Getting your individual fertility picture — AMH for the woman, semen analysis (and possibly DNA fragmentation) for the man — is the most useful thing any couple over 30 can do when trying to conceive and not getting immediate results.

A free fertility assessment maps what investigations make sense for your specific situation.

Age is a variable in the fertility equation. Understanding where you sit is more useful than worrying about where the average person your age sits.