Father’s Diet & BMI Don’t Affect Newborn’s Birth Weight

A recent study published in Nutrients reveals that a father’s body mass index (BMI) and diet quality do not significantly impact a baby’s birth weight or length. The research, conducted as part of the Finnish STEPS Study, contradicts earlier assumptions that paternal health plays a direct role in fetal growth.

Examining the Role of Paternal Health

Previous research has extensively documented maternal factors influencing fetal development, such as diet, placental health, and mental well-being. Maternal obesity before conception increases the risk of child obesity by 264%, and poor nutrition can lead to metabolic changes that persist into adulthood.

However, the father’s contribution to fetal development remains less understood. While animal studies have suggested that paternal factors can influence embryonic development and birth weight, human studies have been lacking.

This new research sought to determine whether paternal BMI and diet directly affect fetal growth.

Study Overview

Researchers analyzed 1,640 father-newborn pairs from the STEPS cohort, which tracked births from 2008 to 2010 in a Finnish hospital district. Paternal BMI and diet quality were assessed at 30 weeks of pregnancy, using the Index of Diet Quality (IDQ). Scores below 10 indicated an unhealthy diet, while scores of 10–15 reflected a healthy dietary pattern.

Key findings include:

• 60% of fathers followed an unhealthy diet, compared to 33.5% of mothers.
• The average paternal BMI was 26, classifying most as overweight.
• No correlation was found between paternal BMI or diet and birth weight or birth length, even after accounting for maternal health factors.

Implications for Fetal Development

This study suggests that paternal BMI and diet do not directly impact fetal growth in terms of size at birth. However, researchers emphasize that this does not rule out potential long-term effects of paternal health on a child’s metabolism and development.

For example, epigenetic factors—modifications in sperm DNA caused by nutrition or environmental exposures—may still influence a child’s long-term health risks, including obesity and metabolic disorders.

Additionally, paternal malnutrition has been linked in previous studies to altered sperm quality, which could affect postnatal growth and disease risk.

Conclusions and Future Research

The findings challenge the conventional wisdom that paternal health directly impacts birth size. However, long-term studies are needed to explore whether paternal BMI and diet influence later-life growth, obesity risk, and metabolic health.

Researchers emphasize that understanding paternal contributions to offspring health remains crucial, as epigenetic and behavioral factors may still play a role in shaping future disease risk.

More Information: Kearns, M. L., Lahdenpera, M., Galante, L., et al. (2025). Association of Paternal BMI and Diet During Pregnancy with Offspring Birth Measures: The STEPS Study. Nutrients. doi: https://doi.org/10.3390/nu17050866.