The “Plus-One” Effect: Why Both Partners’ Ultra-Processed Diets May Be Stalling Pregnancy

March 27, 2026

For decades, the “preconception “to-do list” has been largely a maternal one: take folic acid, quit smoking, and eat your greens. However, a groundbreaking study published on March 23, 2026, in the journal Human Reproduction suggests that when it comes to the dinner plate, the father’s habits may be just as critical as the mother’s.

Researchers at the Erasmus University Medical Center have found that high consumption of ultra-processed foods (UPFs) by both prospective parents is linked to significant fertility challenges and slower early embryonic growth. The study, part of the Generation R Study Next Programme, tracked 831 women and 651 men, providing some of the most robust evidence to date that the “standard modern diet”—rich in instant noodles, sugary sodas, and frozen meals—could be a silent contributor to the global rise in infertility.

The Hidden Cost of Convenience

Ultra-processed foods are defined by the NOVA classification system as industrial formulations typically containing five or more ingredients, such as stabilizers, emulsifiers, and preservatives. In the study, UPFs accounted for approximately 22% of women’s caloric intake and 25% of men’s. While these numbers reflect a moderate intake, they are symptomatic of a global trend where UPFs can comprise up to 60% of daily calories in some high-income nations.

The research team used detailed food frequency questionnaires and early-pregnancy ultrasound scans at 7, 9, and 11 weeks to measure the “crown-rump length” and yolk sac volume of the embryos. The results revealed a distinct “division of labor” regarding how these foods affect reproduction:

• For Men: High UPF intake was strongly associated with a longer “time-to-pregnancy” and an increased risk of subfertility (defined as failing to conceive after 12 months of unprotected sex).

• For Women: While UPF intake did not consistently delay the act of conception itself, it was linked to smaller embryonic size and reduced yolk sac volume by the seventh week of pregnancy.

Why the Father’s Diet Matters

The link between paternal diet and fertility centers on the biological vulnerability of sperm. “In men, higher UPF consumption was related to a higher risk of subfertility and longer duration until pregnancy,” explained study co-author Celine Lin, a PhD candidate at Erasmus MC.

Experts suggest that UPFs trigger oxidative stress and systemic inflammation, which can damage sperm DNA and reduce motility. Dr. Ashok Agarwal, Director of the Andrology Center at Cleveland Clinic (who was not involved in the study), has previously noted that healthy dietary patterns can elevate sperm concentration by as much as 66% compared to diets heavy in processed meats and trans fats.

Furthermore, the paternal diet may have “epigenetic” consequences. This means the father’s nutritional status at the time of conception could potentially “program” the offspring’s future metabolism via markers in the seminal plasma, making the father’s burger-and-fry habit a multi-generational concern.

The Womb Environment and the Yolk Sac

For women, the study’s findings highlight a different concern: the quality of the “first 1,000 days,” beginning at the moment of conception. The discovery of smaller yolk sacs in women with high UPF diets is particularly concerning to neonatologists.

The yolk sac serves as the embryo’s primary nutrient source before the placenta is fully functional. A compromised yolk sac is a known red flag for potential complications, including a higher risk of miscarriage, low birth weight, or even cardiovascular issues later in the child’s life.

“Our findings suggest that a diet low in UPFs would be best for both partners,” says Dr. Romy Gaillard, lead researcher and professor at Erasmus University Medical Center. “It is not only for their own health but also for their chances of pregnancy and the health of their unborn child.”

A Growing Body of Evidence

The Erasmus study does not stand alone. A 2025 analysis of U.S. NHANES data involving over 2,500 women found that those in the highest tier of UPF consumption had 60% lower odds of fertility compared to those who ate whole foods. While some of this was mediated by Body Mass Index (BMI), the underlying chemical burden of UPFs—specifically endocrine-disrupting phthalates found in food packaging—remains a primary suspect in reproductive interference.

Navigating the Limitations

As with any observational study, researchers caution that these findings show an association, not direct proof of cause and effect.

• Confounding Factors: It is difficult to isolate diet from other lifestyle factors such as stress, sleep quality, or environmental pollution.

• Recall Bias: Participants self-reported their eating habits, which can sometimes lead to inaccuracies.

• Small Effect Sizes: While the differences in embryonic growth were statistically significant at a population level, they were small on an individual basis, meaning more large-scale trials are needed to confirm the long-term impact.

Practical Steps for Prospective Parents

For couples navigating the often-stressful journey of conception, these findings offer a tangible area of control. Rather than an overnight dietary overhaul, experts recommend a “swap” strategy.

The “Preconception Audit”

1. Aim for the 15% Rule: Try to keep UPF intake below 10-15% of total daily calories.

2. Focus on Omega-3s: Incorporate fish and healthy fats, which have been shown to improve sperm motility and uterine health.

3. The 90-Day Window: Because sperm production takes about three months, men may see the benefits of a diet change roughly 90 days after making the switch.

By shifting the focus from “maternal responsibility” to a shared “parental partnership,” couples can optimize their reproductive health together—one whole-food meal at a time.

Medical Disclaimer: This article is for informational purposes only and should not be considered medical advice. Always consult with qualified healthcare professionals before making any health-related decisions or changes to your treatment plan. The information presented here is based on current research and expert opinions, which may evolve as new evidence emerges.

References

https://www.earth.com/news/fertility-depends-on-both-parents-diets-not-just-the-mother/