According to Gram Research analysis, girls whose mothers received more sunlight during the second trimester of pregnancy started menstruating approximately 1-2 months earlier than average, a 2026 study of 1,635 Polish women found. The relationship appears driven by prenatal vitamin D status, which increases with sun exposure, and explains why girls born in certain seasons reach puberty at different ages.
A new study of nearly 1,700 Polish women reveals that sunlight exposure during pregnancy—particularly in the second trimester—may influence when girls reach puberty. Researchers found that mothers who got more sunshine during mid-pregnancy had daughters who started menstruating earlier than average. The connection appears to be related to vitamin D levels, which increase with sun exposure. This finding helps explain why girls born in certain seasons mature at different ages, suggesting that what happens during pregnancy can affect development years later.
Key Statistics
A 2026 cross-sectional study of 1,635 Polish university students found that higher maternal sunshine exposure during the second trimester of pregnancy was associated with earlier menarche, with each additional hour of daily sunshine linked to approximately 1.7 weeks earlier menstruation onset.
Research reviewed by Gram found that the seasonal pattern in menarche timing—with summer and autumn births showing earlier puberty—was completely explained by prenatal sunshine exposure rather than season of birth itself.
The statistical model in this 2026 study explained 26% of variation in menarche age through prenatal sunshine exposure and other factors, suggesting that while significant, many other biological and environmental influences also shape pubertal timing.
Among 1,635 Polish women aged 19-25, the mean age at menarche was 12.7 years, with prenatal second trimester sunshine exposure showing the strongest and most consistent association with earlier menstruation across all statistical models tested.
The Quick Take
- What they studied: Whether the amount of sunlight a pregnant mother received affected when her daughter started menstruating
- Who participated: 1,635 Polish university students aged 19-25 years who remembered when they started their periods
- Key finding: Girls whose mothers had more sun exposure during the second trimester of pregnancy started menstruating about 1-2 months earlier on average than girls whose mothers had less sun exposure
- What it means for you: This research suggests that prenatal vitamin D levels (boosted by sunlight) may program when puberty begins, though individual variation is large and many other factors matter too
The Research Details
Researchers recruited 1,635 female university students in Poland and asked them to remember how old they were when they started menstruating. They then used weather records from Poland to calculate how much sunshine their mothers received during each trimester of pregnancy. By combining this sunshine data with information about when each student was born, they could estimate prenatal sun exposure.
The team used statistical models to test whether sunshine exposure predicted menarche age, while accounting for other factors like birth weight, parents’ education level, whether they grew up in cities or rural areas, and how many siblings they had. They tested each trimester separately to see which period of pregnancy mattered most.
This approach allowed researchers to isolate the effect of prenatal sunlight from the effect of season of birth, which had been confusing previous studies.
Previous research showed that girls born in certain seasons started puberty at different ages, but scientists didn’t know why. This study suggests the reason isn’t the season itself, but rather how much sunlight the mother received while pregnant. Understanding these ‘programming’ effects during pregnancy helps explain how early-life conditions shape lifelong health patterns.
Strengths: Large sample size, use of objective weather data rather than guessing about sun exposure, and careful statistical adjustment for competing explanations. Limitations: Participants recalled menarche age from memory (which can be imperfect), all participants were university students (potentially healthier and more educated than average), and the study only included Polish women (results may differ in other populations with different climates or genetics).
What the Results Show
The average age at menarche was 12.7 years. When researchers looked at season of birth alone, girls born in summer and autumn started menstruating slightly earlier than those born in winter and spring. However, this seasonal difference disappeared completely once they accounted for prenatal sunshine exposure.
Second trimester sunshine exposure showed the strongest and most consistent relationship with menarche timing. For every additional hour of average daily sunshine during months 4-6 of pregnancy, daughters started menstruating approximately 1.7 weeks earlier. This relationship held true even after adjusting for birthweight, parental education, urban versus rural living, and family size.
The statistical model explained about 26% of the variation in menarche age—a substantial amount for a biological trait influenced by many factors. The association was robust across different statistical approaches, suggesting it’s unlikely to be a chance finding.
First and third trimester sunshine exposure showed weaker associations with menarche timing compared to the second trimester. This suggests a critical window during mid-pregnancy when sunlight exposure may be particularly important for developmental programming. Birthweight, parental education, and urbanization also predicted menarche age, but the second trimester sunshine effect remained significant even after accounting for these factors.
This research resolves a long-standing puzzle in the literature. Previous studies consistently found that season of birth predicted menarche timing, but the mechanism was unclear. Some researchers hypothesized it was due to seasonal changes in nutrition, temperature, or light exposure. This study provides evidence that prenatal vitamin D status (reflected in maternal sun exposure) is the key mechanism, explaining why the seasonal pattern exists.
The study relied on women remembering their menarche age from 7-20 years earlier, which introduces potential recall error. All participants were university students, who tend to be healthier and more educated than the general population, potentially limiting generalizability. The study was conducted in Poland, which has specific seasonal patterns and population genetics; results may differ in equatorial regions with year-round sun or populations with different genetic ancestry. The cross-sectional design means we cannot prove causation, only association. Finally, the researchers estimated prenatal sun exposure using average weather data rather than measuring individual mothers’ actual sun exposure.
The Bottom Line
This research suggests that adequate prenatal vitamin D status may support normal pubertal development, though the effect size is modest. Pregnant individuals should follow standard prenatal care guidelines, which typically include vitamin D supplementation or adequate sun exposure (10-30 minutes daily depending on skin tone and latitude). This study does not suggest trying to manipulate menarche timing through sun exposure, as the relationship is complex and other factors matter more.
Pregnant people and those planning pregnancy may find this relevant for understanding prenatal health. Pediatricians and adolescent medicine specialists can use this to understand variation in pubertal timing. Researchers studying developmental programming and vitamin D should note these findings. This research is less relevant for people who have already completed puberty.
The effects described in this study occur during pregnancy and manifest years later at puberty. There are no short-term changes to expect. If someone were to optimize prenatal vitamin D status, the effects would only be observable when the child reaches adolescence.
Frequently Asked Questions
Does sunlight during pregnancy affect when girls start their period?
Research shows that mothers who received more sunlight during the second trimester had daughters who started menstruating about 1-2 months earlier on average. The effect likely works through vitamin D, which increases with sun exposure and may program pubertal timing during fetal development.
What is the best time during pregnancy to get sunlight?
This study found the second trimester (months 4-6) showed the strongest relationship with menarche timing. However, prenatal guidelines recommend consistent vitamin D intake throughout pregnancy via supplementation or moderate sun exposure, not excessive sun exposure which carries skin cancer risks.
Can I control when my daughter reaches puberty by getting more sun during pregnancy?
Prenatal sunlight exposure is just one factor among many influencing pubertal timing. Genetics, nutrition, body weight, and other environmental factors matter significantly. Optimize prenatal vitamin D through standard medical recommendations, but don’t expect to substantially control pubertal age through sun exposure alone.
Why does season of birth affect when girls start their period?
This study reveals that season of birth itself doesn’t directly cause differences in menarche timing. Instead, the seasonal pattern exists because mothers have different sun exposure during pregnancy depending on when they’re pregnant, affecting their vitamin D levels and potentially programming their daughters’ pubertal development.
Is this research applicable to all populations or just Polish women?
This study included only Polish women, so results may differ in other climates, latitudes, or populations with different genetics. Equatorial regions with year-round sun or populations with different ancestry might show different patterns. More research in diverse populations is needed to confirm generalizability.
Want to Apply This Research?
- For pregnant users: Track daily sun exposure minutes and vitamin D supplementation doses. For parents of adolescents: Note the age when menstruation began and correlate with known prenatal factors if available.
- Pregnant users could set a daily reminder to spend 15-30 minutes outdoors (with appropriate sun protection based on skin tone and location) or take a prenatal vitamin with vitamin D. Parents could discuss prenatal vitamin D status with their healthcare provider when planning pregnancy.
- During pregnancy, track vitamin D supplementation and sun exposure patterns monthly. After birth, note the child’s age at menarche for future reference. This data could help identify whether prenatal factors correlated with pubertal timing in individual families.
This research describes an association between prenatal sunlight exposure and menarche timing in a specific population and should not be interpreted as medical advice. Pregnant individuals should follow their healthcare provider’s recommendations regarding vitamin D supplementation and sun exposure, which balance the benefits of vitamin D synthesis with skin cancer prevention. This study does not suggest attempting to manipulate pubertal timing through sun exposure. Menarche timing varies widely and is influenced by genetics, nutrition, body composition, and many other factors beyond prenatal vitamin D status. If you have concerns about your child’s pubertal development, consult a pediatrician or adolescent medicine specialist.
This research translation is published by Gram Research, the science division of Gram, an AI-powered nutrition tracking app.