When it comes to infertility, women get a lot of the blame. But the truth is, male fertility decline is a factor in 30–40% of infertility cases. What causes male fertility decline? And how is it similar—and different—from the fertility decline women experience? Here’s a primer!
Male fertility vs. female fertility
Women are born with all the eggs they’ll ever have, known as their egg count or “ovarian reserve.” As women age, they have fewer eggs remaining in their ovarian reserve (they lose about 1,000 eggs each month). Additionally, the eggs that remain are more likely to contain DNA that’s been naturally damaged over time—and that damage can prevent them from fertilizing and resulting in a healthy pregnancy. This is known as reduced egg quality. Because both egg count and egg quality decline with age, it makes sense that—while there are other causes of female infertility—age is the most influential factor when it comes to female fertility.
For men, it’s a bit different. In contrast to the limited, nonrenewable number of eggs women have at birth, male bodies (the testes, specifically) create upwards of 100–200 million fresh new sperm each day. Suffice it to say that men have no risk of “running out” of sperm with age, as is the case with women and their eggs. However, there are a number of factors—including, yes, age—that can affect sperm production or the process of ejaculation, and therefore cause male fertility decline.
What causes male fertility decline
Male fertility decline can be caused by medical issues/injuries, environmental/medical history factors, lifestyle choices, and age. Let’s review the first three, first—we’ll get to age in a moment.
Male fertility decline can be caused by a number of different medical issues that interfere with sperm production/function or block the passage of sperm out of the body (ejaculation). These include:
- Illness, including infections such as epididymitis (inflammation of the epididymis, a duct behind the testes), orchitis (inflammation of the testicles), or some sexually transmitted infections; anti-sperm antibodies, which are immune system cells that mistakenly identify sperm as harmful invaders and attempt to eliminate them; cancers/tumors that affect the glands involved in reproduction (such as the pituitary gland); injury to the testicles or other parts of the reproductive system; or hormone imbalances, such as low testosterone
- Anatomical problems, such as varicocele—a swelling of the veins that drain the testicle that can interfere with sperm production—or problems related to the proper release of sperm into the ejaculate, such as congenital absence of the vas deferens, a condition that results in normal sperm production inside the testicle, but an inability for the sperm to be released into the ejaculate (and therefore an absence of sperm in the semen altogether)
- Chromosomal defects that cause abnormal development of the male reproductive organs
- Problems with sexual intercourse or ejaculation, such as retrograde ejaculation (when semen enters the bladder during orgasm instead of emerging out the tip of the penis), erectile dysfunction, premature ejaculation, or painful intercourse
- DNA fragmentation, meaning that the DNA within the sperm is abnormal, increasing the chance of failed conception or miscarriage
Environmental or medical history factors
Male fertility decline can also be caused by:
- Medications, such as testosterone replacement therapy, cancer medications (chemotherapy/radiation), some antifungal medications or ulcer drugs, and certain other medications that impair sperm production and cause male fertility decline
- Prior surgeries that block the path of sperm, such as hernia repairs, scrotal or testicular surgeries, prostate surgeries, or (obviously) vasectomies
- Environmental factors, such as overexposure to industrial chemicals, heavy metals, or radiation
As in the case of female fertility, there are certain behaviors that can impact the rate of male fertility decline, such as:
- Illicit drug use, such as the use of anabolic steroids, cocaine, or marijuana
- Alcohol use, which can can lower testosterone levels, cause erectile dysfunction, and decrease sperm production
- Smoking, which contributes to a lower sperm count
- Obesity, which can directly impact sperm themselves and/or cause hormone changes that cause male fertility decline
Male fertility decline and age
Women are not the only ones who experience age-related fertility decline, but male fertility decline with age is less significant and less dramatic than what women experience.
Studies demonstrate that, even when controlling for the age of their partners (since older men tend to have older female partners), increasing male age is associated with male fertility decline, defined as increased time to pregnancy and decreased pregnancy rates. In one study of over 8,000 pregnancies, conception in one year was 30% less likely for men over age 40 compared with men under age 30.
In another survey of 1,976 women, an even stronger age effect was found: researchers reported that the average time to pregnancy for men aged 45 years and older was five times greater than the average time to pregnancy for men under age 25. In another study of 901 cycles of intrauterine insemination (a treatment for infertility), men over 35 had pregnancy rates of 25%, compared with pregnancy rates of 52% in men under 35 years. Suffice it to say: age-related male fertility decline is well established by research.
What causes age-related male fertility decline
We’ve established that sperm is renewed daily. So why do we see male fertility rates declining with age?
First of all, as men get older, they have an increased risk of medical conditions that affect sexual health and fertility, such as erectile dysfunction and cancers, as highlighted above. But even perfectly healthy men experience male fertility decline. The research suggests a few potential causes for this decline:
- Decreased sperm motility. “Motility” is the ability of the sperm to move and “swim” properly toward the egg. Research demonstrates that sperm motility decreases with age; one study of healthy, non-smoking men demonstrated that motility decreased .8% per year. Motility is developed as the sperm travel through the prostate and the epididymis, so motility decrease is thought to be due to age-related decline in the function of these glands.
- Decreased sperm morphology. “Morphology” refers to the size and shape of sperm. Studies indicate that normal sperm morphology declines .2–.9% per year, resulting in a 4–18% decrease in normal morphology over a 20-year period. A lower number of normally shaped sperm results in male fertility decline, as sperm with abnormal morphology is less likely to fertilize an egg.
- Decreased seminal volume. There may be a very mild decrease in seminal volume (how much semen is ejaculated) with age. Some reports show a modest change of .15–.2% per year, which accumulates to a 3–4% decrease in seminal volume over a 20-year period (with the most pronounced changes occuring in men over age 45). Some studies, on the other hand, show no difference in volume with age, so the impact this has on male fertility decline is not clear.
How age-related male fertility decline differs from age-related female fertility decline
Unlike low egg quality in women, abnormal DNA in sperm (DNA fragmentation) is not primarily caused by age. Men of any age can experience sperm DNA fragmentation, which is thought to be caused by infection, elevated body temperature (such as a high fever), or lifestyle factors (such as smoking or drug use).
Male fertility declines at a much slower rate than female fertility, with the effects felt later in life. When does male fertility start to decline? Like female fertility, male fertility declines throughout adulthood, but experts suggest that a man’s age doesn’t have a significant effect on his fertility until he’s in his 40s, because there’s only about a 1–2% decrease in sperm motility/morphology per year. That’s a very small change compared to the decrease in egg quality per year for women: at age 38, over 50% of a woman’s embryos are likely to be normal, while at 42, just 20% are likely to be normal, representing a 60% decrease in fertility over 4 years.
And importantly, unlike a woman who reaches menopause when she runs out of eggs, men experience male fertility decline, but don’t become infertile as a result of age. It is possible (though less likely) for men of any age to father a biological child.
Other effects of male age
While age may not be the most important factor when it comes to male fertility decline, there are other important effects of male age on the health of the pregnancy and the child. There’s evidence that the chance of miscarriage increases with paternal age, potentially due to genetic abnormalities in the sperm.
Additionally, the child’s chances of birth defects, schizophrenia, and autism all increase with their father’s age. In one report, offspring of men over 50 were shown to be 2.2 times more likely to have autism than offspring of men under 29. In another, the risk of schizophrenia increased with paternal age, and men over 45 were 2 times more likely to father children with schizophrenia.
The mechanism for this affect is not fully understood yet by researchers. It’s possible that these increased rates of illness are caused by what experts call “de novo mutations,” or a genetic alteration appearing for the first time. However, a 2016 study concluded that de novo mutations are probably a small part of the cause; researchers posited, instead, that men who are genetically predisposed to psychiatric illnesses may also be more likely to delay fatherhood.
Why it’s important to understand male fertility decline
Like women, men are also waiting longer than ever to have children. The average age of the father of a newborn in the United States has risen to 30.9, up from 27.4 in 1972. This trend can be seen worldwide, too: in Germany, the median age of fathers has risen to 33.1, and in England, fathers over 35 accounted for 40% of all births in 2003 (compared with 25% in 1993). Since age is a factor in male fertility decline, it’s important to understand the potential impact of this delay on the ability for older couples to get pregnant and have healthy babies.
Men can preserve their fertility, too
Just like egg freezing can preserve female fertility, sperm freezing is an option for men concerned with male fertility decline. That might include men at risk of injury, such as those in the armed forces, or those who are about to undergo medical treatment—such as chemotherapy—which could affect fertility. And some experts suggest that men who may not become fathers until their 40s or 50s should consider sperm freezing in their 30s, too.
Hooray for proactive health measures!