Case: TitleThe bigger a man’s belly, the more likely embryo problems are? A study of 8,443 people reveals an easily overlooked fertility signal.

When many couples are preparing for pregnancy, most of the attention tends to fall on the woman. How is her AMH? Are there enough follicles? Is the endometrial lining thick enough? What about the quality of her eggs? For the man, the evaluation often stops at a routine semen analysis. As long as the results show nothing obviously abnormal and he does not appear overweight, many people assume the male side is essentially fine.

But the reality may not be that simple. Whether a man looks heavy and where fat actually accumulates are two different questions. The increasingly common “beer belly,” “spare tire,” and abdominal obesity may represent far more than a change in appearance—they may also be associated with sperm concentration, motility, normal morphology rate, sperm DNA integrity, and even embryo quality. This deserves particular attention for couples trying to conceive naturally or preparing for IVF or third-generation IVF in Thailand.

Because reproduction has never been about the egg alone. An embryo receives half of its genetic material from the egg and the other half from the sperm.

Part 1: Data from 8,443 men—weight gain may affect more than just appearance

A 2025 meta-analysis published in Frontiers in Endocrinology systematically examined the relationship between obesity and male reproductive function. From 275 related studies, the researchers further selected 14 that met their inclusion criteria, ultimately enrolling 8,443 men. Participants were divided into normal-weight, overweight, and obese groups based on BMI. The results showed that as weight status changed, multiple semen parameters directly related to male fertility also differed significantly.

Compared with the obese group, normal-weight men demonstrated better sperm normal morphology rate, sperm concentration, progressive motility, and total sperm count. According to the pooled data from the original studies: normal morphology rate P=0.001; sperm concentration P=0.004; progressive motility P<0.0001; total sperm count P<0.0001. Moreover, it was not only semen parameters that were affected. Total testosterone, FSH (follicle-stimulating hormone), LH (luteinizing hormone), and other markers closely tied to male reproductive endocrinology also showed differences between groups.

In other words: the relationship between obesity and male reproductive health is not simply a matter of “being heavier looks less attractive.” It may involve sperm production, motility, morphology, and the reproductive hormonal environment simultaneously. But the story does not end there—because there is an even more easily overlooked indicator than body weight itself.

Part 2: What really deserves attention may not be “how heavy” but “where the fat sits”

When most people judge whether they are overweight, their first thought is BMI. The calculation is likely familiar: BMI = weight (kg) ÷ height (m)². BMI certainly has value, but it has an inherent limitation: it cannot tell us where fat is distributed.

For example, two men of identical height and weight may both have a BMI of 24. One exercises regularly, has a higher muscle ratio, and carries very little fat around the waist and abdomen. The other may have slender limbs but a pronounced beer belly and substantial visceral fat. Despite having the same BMI, their metabolic states can be completely different. This is why male reproductive research has increasingly turned its attention to another measure: WHR—the waist-to-hip ratio. The calculation is equally simple: waist-to-hip ratio = waist circumference ÷ hip circumference

A study published in April 2025 in the International Journal of Molecular Sciences analyzed 543 semen samples, grouping participants by both BMI and waist-to-hip ratio. The findings suggested that an abnormal waist-to-hip ratio may be more clearly associated with unfavorable changes in semen parameters than an abnormal BMI alone. This points to a phenomenon that men preparing for pregnancy should note carefully: some men have a normal BMI, and yet their waistlines are already “raising the alarm.”

Part 3: If he does not look overweight, why is sperm DNA fragmentation elevated?

This is one of the most confusing points for many men. “My weight is normal.” “I’m not even fat.” “So why is my DFI still high?” An important clue from the research is this: don’t just look at body weight—look at abdominal fat.

In the study described above, when participants were grouped by BMI alone, some men of normal weight with higher visceral fat did not show obvious problems in routine semen parameters. However, their sperm DNA fragmentation index had already risen. When the researchers then examined participants by waist-to-hip ratio, the differences became even more pronounced.

In that study’s grouping, men with a waist-to-hip ratio ≥1, compared with those with a waist-to-hip ratio <1, showed lower sperm motility, lower normal morphology rate, lower sperm viability, and a higher sperm DNA fragmentation index.

One detail should be emphasized here: the study used a WHR ≥1 for its exposure-group analysis, which does not mean every clinical scenario must wait until the ratio reaches 1 before attention is warranted. The original article specifically noted that a male waist-to-hip ratio above 0.9 is already worth taking seriously.

So for men preparing for pregnancy: a normal BMI does not automatically mean normal metabolic or reproductive status. If the arms and legs are not heavy but the waist and abdomen protrude noticeably—a beer belly or elevated visceral fat—further attention is warranted.

Part 4: How can a beer belly affect the testicles and sperm?

This is where the science becomes especially worth explaining. Many people used to regard fat as simply the body’s “storage depot” for energy. In fact, that is not the case. Adipose tissue itself has highly active endocrine and metabolic functions. As abdominal fat accumulates, it may affect the male reproductive system through several pathways.

01 | Local temperature changes: the testicles actually “run hot”

Normal sperm production requires a relatively appropriate temperature environment. This is why the testicles are not located inside the abdominal cavity like most organs, but rather in the scrotum. When fat around the abdomen, groin, and surrounding areas increases—combined with prolonged sitting and a lack of exercise—local heat dissipation can be compromised.

Prolonged exposure to an unfavorable temperature environment is not conducive to normal spermatogenesis. For this reason, men trying to conceive are generally advised against frequent sauna use, prolonged hot-spring bathing, or keeping the testicles in consistently high temperatures.

02 | Increased body fat can alter a man’s hormonal environment

Adipose tissue contains aromatase, an enzyme involved in the conversion of androgens to estrogens. When fat tissue increases noticeably, this metabolic environment can shift, potentially disturbing the balance between androgens and estrogens in a man’s body.

Testosterone, in turn, is closely tied to normal sperm production. So abdominal obesity is not merely a matter of “a few extra centimeters around the waist.”

03 | Oxidative stress may further attack sperm DNA

Obesity is often accompanied by chronic low-grade inflammation and increased oxidative stress. Sperm are cells that are particularly sensitive to oxidative stress. When reactive oxygen species are produced in excess of the body’s antioxidant capacity, the sperm cell membrane, mitochondria, and DNA can all be affected.

This helps explain why some men show a relatively normal semen analysis but an elevated DFI. The 2025 review similarly noted that obesity may influence sperm production, motility, and DNA integrity through mechanisms such as testicular inflammation and oxidative stress.

Part 5: Why does this matter especially for families undergoing IVF?

During natural conception attempts, the focus is on whether sperm can successfully fertilize the egg. Once a couple enters an IVF cycle, the evaluation continues further: what about the quality of the embryos formed after fertilization? This is particularly relevant for families preparing for third-generation IVF in Thailand, where blastocyst culture and PGT may follow embryo formation.

The male factor, therefore, should not be set aside once sperm have been successfully retrieved. A multifactorial regression analysis in an IVF population found a statistically significant association between male BMI and the risk of embryonic chromosomal abnormalities. The study reported an OR of 1.132 (95% CI: 1.050–1.220), meaning that for each 1-unit increase in BMI, the associated risk observed in the statistical model increased by approximately 13%. The relationship with trisomy of chromosome 16 drew particular attention from the researchers.

ThaiHo would like to emphasize an important point here: “association” must never be equated with “causation.” This cannot be interpreted as: a heavier man → his embryos will definitely have chromosomal abnormalities. Embryonic chromosomal abnormalities are influenced by a combination of factors, including maternal age, egg quality, sperm quality, and many others.

The more reasonable takeaway from current research is this: when assessing embryo quality, the male partner’s weight, metabolic status, and sperm quality deserve to be included in the overall picture—rather than attributing every problem to female age or egg quality.

Part 6: Already have a beer belly? Is there still a chance to improve?

Yes—and this may be the most important part of this article for men preparing for pregnancy. Another 2025 meta-analysis found that after weight loss, sperm concentration improved significantly, with an effect size of approximately 0.495.

This suggests that for some overweight, obese, or abdominal-obese men, improving body weight and metabolic status through science-based lifestyle management may have a positive impact on semen quality. But be careful not to swing to the opposite extreme: crash-dieting to lose 10 kg in a month before starting IVF.

Extreme calorie restriction, prolonged fasting, or rapid short-term weight loss is not recommended for men during the pre-conception period. The reason is straightforward: the body needs time, and so does sperm. The development of sperm from spermatogonia into mature spermatozoa is not a process that happens in a matter of days.

A full spermatogenic cycle takes approximately 72 to 90 days. Lifestyle adjustments, therefore, are generally recommended to be maintained for at least three months before re-evaluating relevant parameters.

So if there is still preparation time before a fertility journey overseas, cross-border parenthood, or a third-generation IVF cycle in Thailand, it is far more effective to begin systematic adjustments a few months in advance than to load up on so-called “sperm-boosting supplements” in the final week.

Part 7: For men preparing for IVF, these 5 practical steps beat blindly taking supplements

1 | Don’t just weigh yourself—track your waist circumference and waist-to-hip ratio too

Don’t fixate only on the bathroom scale. Men, in particular, should pay attention to: waist circumference, waist-to-hip ratio, and changes in abdominal fat. To measure correctly, stand naturally with the body relaxed, measure the waist at a consistent reference point, and measure the hips at the widest part of the buttocks. Then calculate: waist-to-hip ratio = waist circumference ÷ hip circumference. If the waist and abdomen are increasingly protruding, do not assume “I’m not overweight, so it doesn’t matter”—even if BMI is still within the normal range.

2 | The goal is losing visceral fat, not starving yourself thin

The core of dietary adjustment is not “eating nothing.” Rather, it is gradually reducing: refined carbohydrates, sugary beverages, excessive alcohol, fried foods, takeout, and highly processed foods. At the same time, quality protein from fish, shrimp, eggs, lean meat, and dairy should still be consumed in reasonable amounts. When losing fat during the pre-conception period, faster is not better. What matters is a dietary pattern that can be sustained over the long term.

3 | Aerobic exercise plus strength training is more reliable than last-minute efforts

ThaiHo’s recommendation is: at least 150 minutes per week of moderate-intensity aerobic exercise—such as brisk walking, jogging, swimming, or cycling—along with 2–3 strength-training sessions per week. Aerobic exercise helps reduce visceral fat, while an appropriate amount of strength training helps improve body composition.

During the pre-conception period, however, “the harder the workout, the better” is not the principle either. In particular, prolonged exposure to high-temperature environments should be avoided.

4 | Don’t let prolonged sitting and late nights undo all your efforts

Many men preparing for IVF will diligently take various nutritional supplements, yet sit for more than ten hours a day and go to bed at two or three in the morning. This clearly puts the cart before the horse. A simple rule to adopt: after every 45–60 minutes of sitting, stand up and move around. Also, make every effort to maintain regular, sufficient nighttime sleep.

These habits may not sound as “sophisticated” as expensive health supplements, but they are more fundamental for improving metabolic status, controlling body fat, and reducing oxidative stress. The original research articles specifically listed avoiding prolonged sitting and ensuring adequate nighttime sleep as lifestyle measures that men preparing for pregnancy commonly overlook.

5 | Struggling to conceive or facing repeated failures? Don’t settle for only a basic semen analysis

If any of the following apply—long-term failure to conceive, recurrent biochemical pregnancies, unsatisfactory embryo quality, recurrent implantation failure, or an upcoming IVF cycle—a reproductive physician should assess the male partner based on his specific situation. ThaiHo suggests that after approximately three months of lifestyle adjustment, the following may be re-evaluated in consultation with your physician: semen analysis, sperm DNA fragmentation index (DFI), and relevant reproductive hormones.

One group is especially easy to overlook: men whose routine semen analysis is “basically normal” but who clearly have a beer belly, abdominal obesity, or excess visceral fat. A normal basic semen test does not automatically guarantee that sperm DNA integrity is intact.

A final word: before IVF, don’t let only the woman carry the effort