According to Gram Research analysis, a study of 323 pregnant women found that mothers who exercised regularly during their second trimester had baby girls with approximately 3% less body fat at birth, compared to less active mothers. This benefit appeared only for female infants; baby boys showed no association with maternal exercise levels. The research suggests that prenatal activity may influence infant body composition in a sex-specific way, though additional studies are needed to confirm these findings and understand the underlying mechanisms.
A new study of over 300 pregnant women found that mothers who exercised regularly during their second trimester had baby girls with lower body fat percentages at birth. Interestingly, this benefit didn’t appear for baby boys. Researchers measured how active mothers were during pregnancy and then used a special scanner to measure how much body fat their newborns had within a week of birth. The findings suggest that staying active during pregnancy might help shape a baby’s body composition, though scientists say more research is needed to understand why the effect differs between boys and girls.
Key Statistics
A 2026 cohort study of 323 pregnant women from Western Australia found that highly active mothers had baby girls with 3.2% lower body fat percentage at birth compared to less active mothers, while no such association was observed in baby boys.
Research from the ORIGINS birth cohort showed that newborn girls averaged 13.0% body fat compared to 11.7% in newborn boys, a statistically significant sex difference that was influenced by maternal exercise levels during pregnancy.
According to a 2026 analysis of 323 mother-infant pairs, maternal physical activity during the second trimester was associated with reduced neonatal body fat in female infants (ranging from 1.73% to 3.23% reduction) but showed no effect in male infants.
The Quick Take
- What they studied: Whether pregnant women who exercise more have babies with different amounts of body fat at birth
- Who participated: 323 pregnant women from Western Australia who had babies between 2016 and 2024 (148 boys and 175 girls). Researchers tracked their exercise habits during the middle of pregnancy.
- Key finding: Baby girls born to highly active mothers had about 3% less body fat than girls born to less active mothers. This pattern didn’t show up for baby boys.
- What it means for you: If you’re pregnant or planning to become pregnant, staying active during pregnancy might influence your baby’s body composition, especially if you’re having a girl. However, this is one study, and doctors should guide individual decisions about exercise during pregnancy.
The Research Details
Researchers followed 323 pregnant women in Western Australia and tracked their exercise habits during the second trimester (roughly months 4-6 of pregnancy). They used two different questionnaires to measure how active each woman was, one called the International Physical Activity Questionnaire and another called the Godin Leisure-Time Exercise questionnaire. After the babies were born, within the first week of life, researchers used a special machine called air displacement plethysmography (basically a high-tech scanner) to measure exactly how much body fat each newborn had. This machine is very accurate and works by measuring how much air the baby’s body displaces.
The researchers then used statistical analysis to look for connections between how active the mothers were and how much body fat their babies had. They adjusted their calculations to account for other factors that might affect a baby’s body fat, like the mother’s age, weight before pregnancy, and smoking status.
This type of study is called a cohort study because researchers followed a group of people over time and collected information about them at different points.
Understanding what happens during pregnancy can help us learn how to give babies the healthiest start in life. Body fat percentage in newborns might be connected to health outcomes later in childhood and adulthood. By studying whether maternal exercise affects this, scientists can give better advice to pregnant women about staying active.
This study has several strengths: it measured actual body fat using a precise scientific instrument rather than just estimating it, it included a decent-sized group of participants, and it adjusted for other factors that could affect results. However, the study only looked at one region of Australia, so results might differ in other populations. The researchers also relied on women’s memory and self-reporting of exercise, which can sometimes be inaccurate. The study found an association (a connection) but cannot prove that exercise directly causes lower body fat in babies, other unmeasured factors could be involved.
What the Results Show
The most striking finding was a clear difference between baby boys and girls. Baby girls had more body fat on average (13.0%) compared to baby boys (11.7%), a difference that was statistically significant. When researchers looked at maternal exercise, they found that highly active mothers had baby girls with noticeably lower body fat percentages. Specifically, mothers with high activity levels had daughters with about 3.2% less body fat than mothers with low activity levels. This finding held up when researchers used both exercise measurement questionnaires.
For baby boys, however, the pattern was completely different. Maternal exercise levels during pregnancy showed no meaningful connection to how much body fat the boys had at birth. This sex-specific difference was unexpected and suggests that pregnancy hormones or fetal development might work differently for male and female babies.
The researchers emphasized that these are associations, meaning they found a connection between the variables, but they cannot definitively say that exercise caused the lower body fat in girls. Other factors related to active mothers (like diet, stress levels, or genetics) could also play a role.
The study revealed that baby girls naturally have higher body fat percentages than baby boys at birth, which aligns with some previous research on sex differences in newborn body composition. The consistency of findings across two different exercise measurement tools (IPAQ and Godin questionnaire) strengthens confidence that the association is real and not due to measurement error.
Previous research on maternal exercise during pregnancy has mostly focused on birth weight and gestational diabetes risk. Very few studies have looked specifically at infant body fat percentage, making this research relatively novel. The sex-specific findings are particularly interesting because most pregnancy exercise studies don’t separate results by baby’s sex. This study adds important new information to a gap in the research.
The study only included pregnant women from Western Australia, so results might not apply to all populations worldwide. Researchers relied on women’s self-reported exercise habits, which can be inaccurate, people sometimes overestimate or underestimate how much they exercise. The study measured exercise only during the second trimester, so we don’t know if exercise in other trimesters matters. The study found associations but cannot prove cause-and-effect because many other factors (diet, genetics, stress, sleep) could influence results. Finally, the study only measured body fat at birth, so we don’t know if these differences persist as babies grow.
The Bottom Line
Pregnant women should discuss exercise with their healthcare providers, as current guidelines generally recommend 150 minutes of moderate-intensity activity per week during pregnancy (unless there are medical reasons not to exercise). This study suggests potential benefits for baby girls specifically, but it’s one study and shouldn’t be the only factor in exercise decisions. Confidence level: Moderate, the finding is interesting but needs confirmation from additional research.
Pregnant women and those planning pregnancy should know about this research, especially those interested in optimizing their baby’s health. Healthcare providers counseling pregnant patients about exercise should be aware of these findings. This research is less relevant for people who are not pregnant or planning pregnancy. Anyone with pregnancy complications or medical conditions should follow their doctor’s specific guidance rather than general exercise recommendations.
If maternal exercise does influence baby body composition, the effect would be present at birth (as measured in this study). However, whether these differences in newborn body fat have long-term health consequences is unknown and would require follow-up studies tracking these children over years.
Frequently Asked Questions
Does exercising during pregnancy affect my baby’s body fat?
Research shows that maternal exercise during pregnancy may reduce body fat in newborn girls by about 3%, but this effect wasn’t seen in baby boys. The study measured body fat within days of birth using precise scanning technology, suggesting exercise timing during pregnancy matters.
How much should I exercise while pregnant?
Current guidelines recommend 150 minutes of moderate-intensity activity per week during pregnancy, such as brisk walking or swimming. Always consult your healthcare provider before starting or continuing an exercise program during pregnancy, as individual circumstances vary.
Is it safe to exercise during the second trimester of pregnancy?
For most healthy pregnancies, moderate exercise during the second trimester is safe and beneficial. The second trimester is often considered the most comfortable time for exercise. However, individual medical conditions may require modifications, so discuss your specific situation with your doctor.
Why does maternal exercise affect baby girls differently than baby boys?
Scientists don’t yet fully understand why this sex difference exists. It may relate to different hormone levels, fetal development patterns, or how male and female babies respond to maternal activity. More research is needed to explain this finding.
Will my baby’s lower body fat at birth affect their health later?
This study only measured body fat at birth and didn’t track long-term health outcomes. Whether newborn body fat differences persist or affect childhood health requires additional research following these children over time.
Want to Apply This Research?
- Track weekly exercise minutes during pregnancy (aiming for 150 minutes of moderate activity) alongside trimester markers. Users can log exercise type, duration, and intensity to monitor consistency and identify patterns.
- Set a specific weekly exercise goal during pregnancy (e.g., ‘Walk 30 minutes, 5 days per week’) and use the app to log completed sessions. Create reminders for exercise days and celebrate weekly milestones to maintain motivation.
- Monitor exercise consistency across all three trimesters, not just the second trimester. Track how exercise patterns change as pregnancy progresses and correlate with prenatal visit notes. After birth, the app could track infant health metrics to see if exercise patterns correlate with postpartum recovery.
This research describes associations between maternal exercise and infant body composition but does not establish definitive cause-and-effect relationships. Pregnant women should consult with their healthcare provider before starting, continuing, or modifying any exercise program, as individual medical circumstances vary. This article is for educational purposes and should not replace professional medical advice. The findings apply specifically to the study population and may not generalize to all pregnant women worldwide. Women with pregnancy complications, medical conditions, or high-risk pregnancies should follow their doctor’s specific guidance rather than general recommendations.
This research translation is published by Gram Research, the science division of Gram, an AI-powered nutrition tracking app.
