A semen analysis tells us about sperm count, motility, and shape—but it doesn’t reveal what’s inside. Hidden DNA damage can disrupt embryo development even when results appear normal. Professor Mark Baker explains why sperm DNA fragmentation testing matters and how reducing testicular heat stress could help protect fertility.
When people hear the term “sperm test” or semen analysis, they usually think of the basics: how many sperm are present (count), how well they swim (motility), and how normal they look (morphology). These measures are indeed necessary, but they don’t tell the whole story.
“Inside each sperm cell sits a tightly packed instruction manual—DNA—and a whole toolkit of RNAs and proteins that help kick-start early embryo development,” explains Prof Baker.
DNA is the body’s master instruction manual, guiding cells on how to grow, function, and repair themselves. In sperm, DNA carries half the blueprint needed to make a baby, with the egg providing the other half. The problem? Standard semen analysis doesn’t detect DNA damage, and when damage is present, it can profoundly affect fertility outcomes.
RNA, on the other hand, is made from DNA and helps carry out its instructions. Some RNAs act as messengers, telling cells which proteins to build; others act like tools, helping early embryos switch on the right genes at the right time. In sperm, RNAs don’t make babies on their own, but they provide an essential jump-start until the embryo’s own DNA takes over.
“That inner cargo can be damaged or dysregulated even when a routine semen analysis looks fine, and it can meaningfully affect fertility outcomes,” he explains.
What Sperm DNA Tells Us About Fertility
The DNA inside sperm is a key measure of male reproductive health. When it is intact, the egg can read the instructions and guide embryo development. But when sperm DNA is highly fragmented, the embryo may never get started. This can make conceiving naturally more difficult and can affect fertilization and embryo growth even with assisted techniques like IVF or ICSI.
Embryos made with sperm carrying a lot of DNA damage are more likely to stall in the earliest pregnancy stages or miscarry. Research shows that higher levels of sperm DNA fragmentation are linked to a greater risk of miscarriage, even when the egg and uterus appear healthy.
What is Sperm DNA Damage?
“Think of sperm DNA as a zip file containing baby-making instructions. If the zip file has broken bits or scrambled code, the embryo may fail to start, stall early, or misread instructions later,” Prof Baker explains.
This problem, called sperm DNA fragmentation (SDF), does not change what sperm look like under a microscope, but it can drastically affect how well they function.
Why Look Beyond Count and Motility?
Classic semen tests can miss hidden problems.
“Many men with unexplained infertility or recurrent pregnancy loss have normal counts and motility, yet elevated SDF—information a standard semen analysis cannot see,” Prof Baker says.
Studies following men with fertility issues show that those whose partners have had repeated miscarriages often have higher levels of sperm DNA damage, even when their sperm count, movement, and shape appear normal. Recognising this, professional guidelines now support sperm DNA testing in certain cases, such as repeated miscarriages, multiple failed IVF attempts, or unexplained infertility.
Testing can uncover hidden DNA breaks that are linked to poorer embryo development and higher miscarriage rates. It can also identify actionable risk factors, such as heat exposure, which can be modified to protect sperm DNA. “It gives a better ‘why’ for unexplained infertility, helping clinicians focus on interventions that reduce sperm DNA damage instead of repeating the same treatment cycle,” he adds.
What It Means for You
In today’s urban environment, sperm are frequently exposed to heat from work or lifestyle preferences such as saunas or hot tubs. Because sperm are highly sensitive to temperature, this can increase DNA fragmentation and reduce sperm production. The encouraging news is that many of these effects are reversible once the heat source is removed.
SDF information can also help refine fertility treatment plans.
“Elevated SDF has been linked to poorer embryo development in IVF/ICSI; knowing this can shape decisions such as treating conditions like varicoceles, adjusting timing after fever, or addressing modifiable risks before another cycle,” he advises.
“What’s driving our interest is that sperm DNA occurs when the testis is exposed to hyperthermia or testicular heat stress. This is surprisingly common causation, and more importantly, preventable. Currently, we are pursuing the use of a scrotal cooling device to help men improve not just semen quality but also reduce the level of sperm DNA damage.”
The Bigger Picture
Fertility care has long focused on egg quality, but sperm quality is more than numbers on a lab report. Inside every sperm cell is a delicate package of DNA that must remain intact to give a new life the best possible start.
Routine semen analysis is a useful first step, but it can miss hidden vulnerabilities that matter deeply for couples struggling with infertility or miscarriage. Testing for sperm DNA fragmentation adds a new layer of precision—offering not just answers, but actionable steps. From lifestyle changes to targeted medical interventions, protecting sperm DNA can make a tangible difference in reproductive outcomes.
Reference:
¹ Alvarez Sedó C, Bilinski M, Lorenzi D, Uriondo H, Noblía F, Longobucco V, Lagar EV, Nodar F. Effect of sperm DNA fragmentation on embryo development: clinical and biological aspects. JBRA Assist Reprod. 2017 Dec 1;21(4):343-350. doi: 10.5935/1518-0557.20170061. PMID: 29116706; PMCID: PMC5714603. https://pmc.ncbi.nlm.nih.gov/articles/PMC5714603/
