Surprising study reveals dads’ weight and diet don’t impact baby’s size

By Dr. Liji Thomas, MDReviewed by Lily Ramsey, LLMMar 4 2025

Experts once thought a father’s health influenced birth weight—but new research says otherwise. Here’s what it really means for your baby.

Study: Association of Paternal BMI and Diet During Pregnancy with Offspring Birth Measures: The STEPS Study. Image Credit: VGstockstudio/Shutterstock.com

Birth weight and length are strongly affected by the mother’s body mass index (BMI) and diet. Some evidence from animal studies suggests an important role for paternal health measures also.

A recent study published in Nutrients looked for such associations in humans, concluding that the father’s body weight or diet does not affect the birth weight or length of the baby.

Introduction

Overweight and obesity are taking a heavy toll on public health worldwide. These conditions are associated with an increased risk of diabetes, metabolic syndrome, cardiovascular disease (CVD) such as strokes, heart disease, kidney disease, liver and gallbladder disease, and some cancers.

Contributing factors may operate in fetal life, especially for diabetes and obesity. This emphasizes the need to understand how these mechanisms work.

Both birth weight and birth length affect neonatal health and death rates in this period. They are also associated with cardiovascular and metabolic diseases in later life. The mother’s dietary habits, placental health, and mental health all play a part in proper nutrient transfer to the fetus.

Malnutrition during crucial periods of early development may leave their imprint on metabolism in adult life. Obesity before conception increases the chances of child obesity by 264%. However, the role played by paternal weight and diet is unclear, relying mostly on animal studies,

These have shown that DNA and embryonic changes, both structural and metabolic, are coupled with effects on birth weight in neonatal and adult life. Birth length may be associated with bronchopulmonary dysplasia and is inversely correlated with overweight/obesity in adolescent males.

About the study

The current study focused on examining whether paternal BMI and diet quality affected fetal growth. The data came from the Finnish “Steps to Healthy Development of Children” (STEPS) study, including mothers who bore live children between 2008 and 2010 in a single Finnish hospital district, with the fathers of the children.

This is the first study to explore this area in a Finnish cohort.

It included 1,640 fathers with BMI or dietary data and 1,640 newborns.  Diet quality was assessed at 30 weeks of pregnancy using the Index of Diet Quality (IDQ). Scores below 10 were considered unhealthy/nonadherence to dietary guidelines, while 10-15 points indicated a healthy diet/adherence.

Study findings

The mean paternal age was 33 years, and the average BMI was 26 kg/m2. The average IDQ was 8.9, compared to 10.2 for mothers. Most children were born at term.

Surprisingly, 60% of fathers ate an unhealthy diet, compared to 33.5% of mothers. The mean paternal BMI indicates that they were often overweight.

The paternal BMI and dietary quality score showed no correlation with either birth weight or birth length, even after compensating for the mother’s weight and diet. 

Implications for fetal development

The paternal contribution to fetal environmental exposures requires further study. Conventional wisdom attributes adverse fetal developmental effects within and across generations to reduced paternal diet quality.

A deficiency of methyl donors like folate from green leafy vegetables and fruits, legumes, and beans has been linked to birth defects in mouse offspring.

Early pregnancy growth restriction may occur because of maternal malnutrition, smoking, or genetic factors. Paternal malnutrition may cause epigenetic changes in the sperms, affect semen quality, and reduce postnatal growth and weight in the offspring. These offspring may grow up to have metabolic disease as adults.

Despite the lack of association between paternal BMI and diet, and the birth weight and length of the offspring, longitudinal follow-up could conceivably reveal associations with later growth rates and obesity or overweight in adolescence and adult life.

These may be mediated by epigenetic changes and other factors that disrupt the normal growth and development of various organs and body systems. The ultimate outcome is a change in adult metabolism, behavior, and choices.

Conclusions

The findings of this study suggest that paternal BMI and diet quality do not significantly affect either birth length or weight. However, long-term follow-up is indicated to capture more long-term results of these paternal factors on offspring metabolic health and behaviors.

“A better understanding of paternal influences on offspring growth and development, including health outcomes, could have important implications for public health.”