New findings published in the peer‑reviewed journal Human Reproduction suggest that diets high in ultra-processed foods, often called UPFs, are linked to poorer reproductive outcomes in men and subtle but measurable changes in early embryonic development in women.
The research points to reduced fertility in men, slower embryonic growth, and smaller yolk sacs in early pregnancy. These effects may be small in individual cases, yet researchers say they could matter greatly when considered across populations.
Ultra-processed foods are now a dominant feature of modern diets. They include packaged snacks, sugary drinks, ready meals, processed meats, and many convenience foods designed for long shelf life and rapid consumption.
The findings for men were clearer in relation to fertility itself. Higher intake of ultra-processed foods was linked to a lower probability of conceiving within a given month and a greater risk of subfertility. Men with diets heavier in UPFs tended to take longer to achieve pregnancy with their partners
Such products are typically high in added sugars, salt, unhealthy fats, and chemical additives, while lacking fibre, vitamins, and whole food ingredients. In many high‑income countries, these foods account for more than half of daily energy intake.
Their impact on long‑term health has been widely discussed. Their potential influence on fertility and early embryonic growth has, until now, remained largely unexplored.
The study draws on data from a large population‑based research programme in the Netherlands that has been following families from before conception through pregnancy and into childhood. Researchers analysed information from more than 800 women and over 600 male partners who were enrolled either before conception or in early pregnancy. Dietary habits were assessed using detailed questionnaires completed around the twelfth week of pregnancy, capturing what participants had been eating in the period close to conception.
Each reported food item was classified according to the degree of processing. Ultra-processed foods were calculated as a percentage of total daily intake by weight. On average, UPFs made up just over one fifth of women’s diets and about one quarter of men’s. These figures reflect typical consumption levels in many European countries, lending the findings broader relevance.
Fertility outcomes were assessed using self‑reported time to pregnancy, the likelihood of conceiving within a month, and whether couples experienced subfertility, defined as taking more than a year to conceive or requiring assisted reproductive techniques. For those who became pregnant, early embryonic development was tracked using high‑resolution transvaginal ultrasound scans at seven, nine, and 11 weeks of gestation.
Two key markers were measured. The first was crown‑rump length, the distance from the embryo’s head to its bottom, which serves as a standard indicator of growth and developmental progress in early pregnancy. The second was the size of the yolk sac, a structure that plays a vital role in nourishing the embryo before the placenta is fully formed.
When researchers examined maternal (mother’s) diets, the picture that emerged was nuanced. Higher consumption of ultra-processed foods in women was not consistently linked to longer time to pregnancy or a higher risk of subfertility. However, it was associated with slightly smaller embryos and reduced yolk sac volume as early as the seventh week of gestation. These differences persisted after accounting for factors such as age, body mass index, smoking, and overall energy intake.
Scientists emphasise that the observed differences in embryonic size were modest. They would not be noticeable in individual pregnancies. From a scientific and public health perspective, though, they are important. Early embryonic growth sets the foundation for later development. Even small shifts, when seen across thousands or millions of pregnancies, could translate into meaningful differences in health outcomes.
The findings for men were clearer in relation to fertility itself. Higher intake of ultra-processed foods was linked to a lower probability of conceiving within a given month and a greater risk of subfertility. Men with diets heavier in UPFs tended to take longer to achieve pregnancy with their partners. Interestingly, paternal diet was not associated with early embryonic growth once pregnancy occurred, suggesting that the primary effect may act before conception.
Experts involved in the study suggest several possible explanations. Sperm cells are known to be highly sensitive to nutritional status, oxidative stress, and inflammation. Diets high in ultra-processed foods may affect sperm quality through poor nutrient composition, excess sugars and fats, or exposure to food additives and packaging‑related chemicals.
In contrast, maternal diet may influence the biochemical environment of the uterus from the earliest days after conception, shaping how the embryo grows and how supporting structures such as the yolk sac develop.
The yolk sac itself deserves attention. Although temporary, it is essential in early pregnancy. It provides nutrients, supports blood formation, and helps regulate early metabolism before the placenta takes over. Previous research has shown that abnormal yolk sac development is linked to miscarriage, premature birth, and other complications. Smaller yolk sac size, as observed in women with higher UPF consumption, may therefore be a subtle signal of altered early development.
Other studies have already established that slower growth in the first trimester is associated with increased risks later on. These include preterm birth, low birth weight, and a higher likelihood of cardiovascular problems during childhood. The current research does not track children long enough to assess such outcomes, but it raises important questions about how early dietary exposures might echo across the life course.
Researchers are careful to point out the limitations of their work. The study is observational, meaning it can identify associations but cannot prove cause and effect. Dietary data were collected using questionnaires, which rely on self‑reporting and may involve some inaccuracy. The participants were drawn from a relatively affluent population, which may limit generalisation to other settings with different dietary patterns or socioeconomic conditions.
Despite these caveats, the study breaks new ground by examining the combined influence of both parents’ diets on fertility and early embryonic development. Most previous research has focused almost exclusively on mothers. This work adds to a growing body of evidence suggesting that paternal health and lifestyle before conception play a critical role in reproductive success.
Scientists involved in the research argue that fertility and early pregnancy should be viewed as a shared biological process, shaped by the health of both partners. Focusing solely on women risks overlooking half of the equation. The findings support calls for broader preconception health strategies that include men, addressing diet, weight, smoking, and other modifiable factors well before couples start trying for a baby.
The study also contributes to the wider debate about ultra-processed foods. While these products are often defended for their convenience and affordability, evidence continues to mount about their potential downsides. Links have already been established with obesity, type 2 diabetes, cardiovascular disease, and certain cancers. The possibility that they may also influence fertility and the earliest stages of human development adds another layer of concern.
Researchers suggest several biological mechanisms that warrant further investigation. One is nutrient displacement. Diets high in UPFs tend to be low in essential micronutrients such as folate, iron, and omega‑3 fatty acids, all of which are important for reproductive health.
Another is inflammation. Ultra-processed foods have been linked to chronic low‑grade inflammation, which could impair sperm quality or disrupt early embryonic environments. Exposure to additives, plasticisers, and microplastics through food packaging is another area of growing interest.
Future studies aim to replicate these findings in more diverse populations and to explore whether reducing ultra-processed food intake before and during pregnancy can lead to measurable improvements in fertility and early development. Long‑term follow‑up of children will also be crucial to determine whether the early differences observed translate into lasting health effects.
For now, the message from researchers is cautious but clear. Diet matters, not just during pregnancy, but in the months leading up to conception. Choosing diets richer in minimally processed foods, such as vegetables, fruits, whole grains, legumes, nuts, and unprocessed proteins, may support reproductive health for both men and women.
The image of the tiny embryo, carefully measured and studied, serves as a powerful symbol. It highlights how early life begins and how sensitive it may be to the choices made even before pregnancy is confirmed. As ultra-processed foods continue to dominate modern diets, understanding their broader impact becomes increasingly urgent.
This study suggests that the effects may reach further back, and deeper, than previously assumed.
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