Prenatal sunlight in the second trimester predicts timing of menarche in Polish women born between 1985 and 1991

Abstract

Season of birth has long been associated with variation in the timing of sexual maturation. However, the specific prenatal environmental mechanisms underlying this relationship remain unclear. We aimed to examine the association between prenatal maternal sunshine exposure and age at menarche, accounting for season of birth and relevant biological and socioeconomic factors. The study included 1,635 Polish female university students aged 19–25 years. Age at menarche was retrospectively reported. Prenatal sunshine exposure was estimated using national meteorological data on average monthly sunshine duration in Poland and calculated separately for each trimester of pregnancy. Generalized linear models assessed associations between trimester-specific sunshine exposure, season of birth, and age at menarche, with progressive adjustment for birthweight, parental education, urbanization level, number of siblings, and birth order. Sequential multiple regression analyses evaluated independent trimester effects. The mean age at menarche was 12.70 ± 1.17 years. In unadjusted analyses, summer and autumn birth were associated with earlier menarche; however, these associations were fully attenuated after adjusting for prenatal sunshine exposure. Higher maternal sunshine exposure during the second trimester was consistently and robustly associated with earlier menarche across all models (e.g. for the final model β=-0.145, 95% CI -0.21, -0.08; adj. R² = 0.256). Prenatal maternal sunshine exposure, particularly during the second trimester of pregnancy, is a significant predictor of age at menarche independent of season of birth. These findings strongly support the role of gestational sunlight exposure - likely reflecting maternal vitamin D status - in the developmental programming of female pubertal timing.

Introduction

The influence of seasonality - particularly month or season of birth - on human health and somatic characteristics has been extensively documented in the scientific literature over recent decades. Significant associations have been reported for a wide range of biological and health-related traits, including e.g., height (Waldhoer et al. 2025), body mass index (BMI) (Lv et al. 2015), and cardiovascular disease (Reffelmann et al. 2011). Furthermore, epidemiological studies indicate that the month of birth may be associated with reproductive parameters, including age at menopause (Cagnacci et al. 2005), mean number of offspring (Huber et al. 2008), and age at menarche (Day et al. 2015; Kliś et al. 2016).

Taken together, these findings provide strong support for the foetal origins of adult disease hypothesis, which posits that intrauterine exposures can have lasting effects on health trajectories later in life (Fall 2013). Several mechanisms have been proposed to explain season-of-birth effects, including seasonal variation in maternal exposure to environmental factors such as sunlight (Siniarska and Kozieł 2010), air pollution (Kokkinari et al. 2025), and food availability (Prasad et al. 2010).

Despite its biological and clinical relevance, age at menarche remains relatively underrepresented in research on birth seasonality, even though the timing of pubertal onset has been consistently linked to a range of adverse health outcomes, including metabolic disorders (Rahimi et al. 2022), and cardiovascular disease (Okoth et al. 2020). Numerous biosocial and environmental determinants of menarcheal age have been identified, including birthweight (Adair 2001), current body weight, height, and BMI (Shrestha et al. 2011), as well as family size, composition, and birth order (Steppan et al. 2019). Most studies have found that living in urban areas (Adair 2001; Padez 2003; Chavarro et al. 2004; Wronka and Pawlińska-Chmara 2005), having parents with higher educational attainment (Hossain et al. 2010) and of higher social class (Behel and Raje 2021) as well as earlier birth order (Morris et al. 2010) and smaller family size (Krzyżanowska et al. 2016) have been found to associate with earlier age at menarche.

Recent evidence suggests that the association between season of birth and age at menarche is primarily mediated by environmental factors influencing foetal development (Day et al. 2015). A key factor in this context is sunshine duration, which varies both seasonally and geographically. Ultraviolet radiation exposure and latitude have been linked to the timing of menarche (Dossus et al. 2013). Meteorological data from Poland indicate a long-term increase in annual sunshine duration over recent decades, accompanied by substantial interannual variability (Bartoszek et al. 2021).

Concurrently, growing attention has been directed toward the role of vitamin D in human growth and health, including its influence on female reproductive physiology (Dragomir et al. 2024). In humans, the primary source of vitamin D is endogenous cutaneous synthesis stimulated by exposure to UV-B radiation. Previous studies conducted in Poland suggested an association between season of birth and age at menarche; however, this relationship was observed only among women born between 1982 and 1984 (Kliś et al. 2016).

Given that numerous studies have demonstrated pronounced seasonal variation in maternal circulating levels of 25-hydroxyvitamin D [25(OH)D] (Maugeri et al. 2019; Liu et al. 2024) - a biomarker reflecting sunlight exposure and a determinant of foetal vitamin D status, the present study aims to examine the relationship between maternal sunshine exposure and age at menarche. Importantly, this relationship is evaluated while accounting for key confounding factors, including birthweight, socioeconomic conditions, and season of birth.

Materials and methods

The research was conducted in 2010 on a sample of 1,920 Polish female university students aged 19–25 years (born between 1985 and 1991) at the University of Wrocław. Approximately 40% of the participants originated from Lower Silesia, the region where Wrocław is located, while the remaining 60% came from other parts of Poland. Therefore, the study sample represented women from diverse geographical regions across the country.

Statistical analyses were performed on a sample of 1,635 women for whom complete information on age at menarche, birth parameters, socioeconomic status, and age at menarche of the students’ mothers was available. The selection process of the subjects is presented in Fig. 1.

Fig. 1.

Flow chart of the selection process

Data were collected in accordance with the ethical principles stated in the Declaration of Helsinki. Informed consent was obtained from all individual participants included in the study. Age at menarche was assessed based on the date provided by the subjects. Participants were asked to provide the date of their first menstruation with accuracy within one month. The age at menarche was calculated from the date of birth and the date of menarche. In cases where women reported only the month and year of menarche, the 15th day of the indicated month was used for calculation. When the date of menarche was reported approximately in the range of 2–3 months, the midpoint of the reported period was used for calculation.

Maternal sunshine exposure was estimated using Polish recorded data from the Institute of Meteorology and Water Management, National Research Institute, based on the average monthly sunshine duration for the whole country. For each individual, the prenatal cumulative hours of sunshine duration were calculated and grouped by trimester (including the month of birth), using monthly averages for the whole country. The average sunshine duration for each trimester and the entire pregnancy was calculated (using the sum of sunshine hours during corresponding months). Due to a lack of data on the precise locations of mothers’ residences during pregnancy, the average sunshine duration for the whole of Poland was used. Additionally, the cumulative sunshine duration during pregnancy, including the month of birth, was calculated as the sum of the average sunshine duration for each pregnancy month.

Socio-economic status was included in the analysis as covariates based on the father’s and mother’s education, the degree of urbanization of the place of residence during childhood and adolescence, the number of siblings, and birth order. The following categories were created: for father’s and mother’s education: primary or vocational, secondary, higher; for the place of residence in childhood and adolescence: village or small town, city above 100,000 inhabitants; for number of siblings: none, one, two or more; for birth order: first born, second born, third born or subsequent.

Statistical analyses were performed using SPSS ver. 29.0.2.0 (IBM SPSS Statistics). Descriptive statistics (arithmetic mean and median, skewness and kurtosis with their standard errors) and the Shapiro-Wilk test were used to assess the normality of the distribution of the quantitative variables. Antenatal sunshine exposure in each trimester was analysed as a quantitative variable. In the statistical analyses, general linear model (GLM) with Bonferroni correction were undertaken. The analyses were carried out in two stages. In the first stage, the relationship between season of birth and age of menarche was analysed using two models: unadjusted and adjusted for estimated mean sunshine exposure across all three trimesters. Then, four further models were undertaken to analyse the relationship between age at menarche and estimated mean sunshine exposure, where model 1 included the first trimester in relation to age at menarche, the second trimester in relation to age at menarche, and so on. Model 2 was adjusted for birthweight. Model 3 was adjusted for birthweight and season of birth. Model 4 was adjusted for birthweight, season of birth, and all socio-economic variables: father’s and mother’s education, degree of urbanization of the place of residence, number of siblings, and birth order. To assess the effect of maternal sunshine exposure on age at menarche after controlling for the effects of all other independent variables, before testing for the variable of choice, the sequential multiple regression was applied as a model 5. For example, the independent association between age at menarche and mean sunshine exposure during the first trimester was assessed after removing the effects of mean sunshine exposure during the other two trimesters, birthweight, season of birth, and all socio-economic variables.

All associated variables were selected based on the literature on the subject. Significance in all statistical tests was set at the level of at least p ≤ 0.05. Descriptive characteristics of the subjects are presented in Table 1.

Table 1.

Descriptive characteristics of the subjects

	N (%)	Mean	SD	Min-Max
Biological traits
Age	1635 (100)	22.87	1.86	18.39–28.43
Age at menarche	1635 (100)	12.70	1.17	9.92–16.20
Birthweight	1635 (100)	3299.60	505.80	1400–5400
Height	1635 (100)	166.94	5.80	145.0-193.0
Weight	1633 (99.9)	59.59	7.81	50.0-110.0
BMI	1633 (99.9)	21.37	2.48	16.33–31.68
Estimated sunshine exposure and season of birth
1st Trimester	1635 (100)	130.48	64.13	30.66-274.74
2nd Trimester	1635 (100)	130.17	61.14	30.66-274.74
3rd Trimester	1635 (100)	130.65	64.16	32.37-276.68
	Category		N	%
Season of birth	Spring		415	25.4
	Summer		407	24.9
	Autumn		386	23.6
	Winter		427	26.1
Socioeconomic variables
Father’s education	Primary or Vocational		427	26.1
	Secondary		723	44.2
	Higher		485	29.7
Mother’s education	Primary or Vocational		306	18.7
	Secondary		825	50.5
	Higher		504	30.8
Urbanization	Rural		518	31.7
	Urban		1117	68.3
Number of siblings	0		282	17.2
	1		748	45.8
	2 or more		605	37.0
Birth order	1		861	52.7
	2		554	33.9
	3 or subsequent		220	13.4

Results

Mean age at first menstruation was 12.70 years with a SD of 1.17 (ranged from 9.92 to 16.20 years); the median for the group was 12.67. Season of birth was associated with reported age at menarche in women; individuals born in autumn (p < 0.001) and summer (p = 0.036) were more likely to have the first menstruation earlier than those born in spring. After adjusting for estimated mean sunshine exposure during all three trimesters, there was no significant relationship (Table 2).

Table 2.

Age at menarche (mean ± standard deviation) in relation to season of birth. GLM models with post-hoc Tukey HSD test; 1-unadjusted; 2-model adjusted for estimated antenatal sunshine exposure

Season of birth	Age at menarche	GLM
		Model 1 with post-hoc test		Model 2
Spring (1)	12.84 ± 1.17	F = 3.800 p = 0.009	(1)-(2) 0.036 (1)-(3) < 0.001	F = 1.983 p = 0.115
Summer (2)	12.67 ± 1.16
Autumn (3)	12.56 ± 1.20
Winter (4)	12.72 ± 1.13

Spring (March-April-May), Summer (June-July-August), Autumn (September-October-November), Winter (December-January-February)

Estimated sunshine exposure during the second trimester was independently associated (p < 0.001 in each model) with age at menarche. An additional third trimester effect was observed only for the unadjusted model (p = 0.03) (Table 3; Fig. 2).

Table 3.

Age at menarche (*regression coefficients) in the Polish young women in relation to estimated antenatal sunshine exposure. GLM models: 1-unadjusted; 2-model adjusted for birthweight; 3-model adjusted for birthweight and season of birth; 4-model adjusted for birthweight, season of birth, and socio-economic factors (parents’ education, dwelling place, number of siblings, and birth order); 5-model adjusted for the other two trimesters, birthweight, season of birth, and socio-economic factors. In Model 1 R² is given, whereas in other models adjusted R²

Antenatal sunshine exposure	Age at menarche*	GLM
		Model 1	Model 2	Model 3	Model 4	Model 5
1st Trimester	0.00057	p = 0.204; R2 = 0.001 β = 0.03 (95% CI -0.02, 0.08)	p = 0.195; R2 = 0.001 β = 0.03 (95% CI -0.02, 0.08)	p = 0.416; R2 = 0.005 β = 0.03 (95% CI -0.04, - 0.10)	p = 0.795; R2 = 0.245 β=-0.01 (95% CI -0.07, 0.05)	p = 0.154; R2 = 0.256 β=-0.06 (95% CI -0.14, 0.02)
2nd Trimester	-0.00293	p < 0.001; R2 = 0.03 β=-0.16 (95% CI -0.20, - 0.10)	p < 0.001; R2 = 0.024 β=-0.16 (95% CI-0.20, - 0.11)	p < 0.001; R2 = 0.024 β=-0.20 (95% CI -0.27, - 0.13)	p < 0.001; R2 = 0.255 β=-0.14 (95% CI -0.21, - 0.08)	p < 0.001; R2 = 0.256 β=-0.14 (95% CI -0.21, - 0.08)
3rd Trimester	-0.00098	p = 0.030; R2 = 0.003 β=-0.05 (95% CI-0.10, - 0.01)	p = 0.029; R2 = 0.002 β=-0.05 (95% CI -0.10, - 0.01)	p = 0.048; R2 = 0.007 β=-0.10 (95% CI -0.20, -0.01)	p = 0.138; R2 = 0.246 β=-0.06 (96% CI -0.15, 0.02)	p = 0.028; R2 = 0.256 β=-0.12 (95% CI -0.24, - 0.01)

Fig. 2.

Age at menarche and sunshine duration during pregnancy trimesters. *** p < 0.001 in Models 1 to 5; *p = 0.030 in Model 1

Discussion

In the present study, we investigated the associations between season of birth, prenatal sunlight exposure, and age at menarche in a large cohort of Polish female university students. All analyses were adjusted for key biological and socioeconomic covariates, including birthweight, birth order, and a comprehensive set of indicators reflecting early-life socioeconomic conditions. Prenatal exposure to sunlight emerged as a robust predictor of pubertal timing and remained significantly associated with age at menarche after controlling for season of birth and other confounders. In particular, a strong inverse association was identified between sunlight exposure during the second trimester of pregnancy and age at menarche, indicating earlier pubertal onset among individuals exposed to higher levels of prenatal sunlight.

Seasonal variation is widely recognized as an essential determinant of prenatal development and long-term health outcomes (Cozzani et al. 2023). Environmental factors that vary seasonally, such as ambient temperature and photoperiod, have been shown to influence foetal growth trajectories, with potential consequences for later health status and reproductive function (Day et al. 2015; Kliś et al. 2016). These effects are typically attributed to season-dependent differences in maternal sunlight exposure, air quality, physical activity patterns, and nutritional intake (Haggarty et al. 2013).

Among these factors, maternal exposure to sunlight during pregnancy is considered one of the most influential environmental determinants of foetal and postnatal development (Waldie et al. 2000). Sunlight exposure plays a central role in regulating circulating concentrations of 25-hydroxyvitamin D [25(OH)D] in pregnant women and, consequently, in the developing fetus (Liu et al. 2024). Vitamin D deficiency during pregnancy has been associated with a range of adverse maternal and neonatal outcomes, including an increased risk of preterm birth, pre-eclampsia, gestational diabetes, and the delivery of infants small for gestational age (Miliku et al. 2016; Fogacci et al. 2020).

Because fetal vitamin D stores are entirely dependent on maternal supply, infants born to vitamin D–deficient mothers are at an increased risk of deficiency at birth (Kozgar et al. 2020). Severe vitamin D deficiency in cord blood has been linked to elevated risks of preterm delivery, neonatal respiratory distress syndrome, increased hospitalization during the first year of life, and poorer neurodevelopmental outcomes (Treiber et al. 2020; Voltas et al. 2020). Evidence regarding the role of maternal vitamin D status in shaping prenatal and postnatal growth remains mixed. Several studies have reported associations between low maternal vitamin D concentrations during pregnancy and reduced neonatal anthropometric measures (Maugeri et al. 2019), lower tibial bone mineral content at birth (Viljakainen et al. 2011), and an increased risk of excessive weight gain in early childhood (Morales et al. 2015). However, other investigations have failed to confirm these findings (Wang et al. 2018), highlighting the complexity of vitamin D–related pathways and the influence of additional modifying factors. Despite these inconsistencies, numerous studies have demonstrated a high prevalence of vitamin D deficiency among pregnant women worldwide, including in regions with abundant solar radiation (Woon et al. 2019).

Season of birth is commonly used as a proxy measure for multiple prenatal environmental exposures, including maternal and foetal vitamin D status, and has been associated with a range of reproductive outcomes, such as age at menarche (Day et al. 2015), fertility in both women and men (Boland et al. 2020), and age at menopause (Cagnacci et al. 2005). Studies conducted in populations residing in the Northern Hemisphere have consistently reported earlier menarche among women born during the summer months (Day et al. 2015; Kliś et al. 2016). Our results are in line with these observations, as the lowest mean age at menarche was observed among participants born in summer and autumn. Importantly, however, the effect of season of birth was no longer statistically significant after inclusion of prenatal sunlight exposure in the analytical models, suggesting that sunlight exposure during gestation represents a more proximal and biologically relevant determinant of pubertal timing.

Although substantial geographic variation in the timing of sexual maturation has been documented, the biological mechanisms underlying this phenomenon remain incompletely understood. One leading hypothesis implicates differences in ultraviolet radiation exposure, which decreases with increasing latitude. Evidence from the French E3N cohort indicates that women born in regions with higher sunlight exposure—and consequently higher UVB radiation—experienced menarche approximately 3–4 months earlier than those born in areas with lower UV exposure (Dossus et al. 2013). Moreover, an increase of 1 kJ/m² in annual UVB dose was associated with a 0.42-year reduction in age at menarche, independent of other predictors.

A critical unresolved issue concerns the specific prenatal period during which sunlight exposure - and, by extension, maternal vitamin D status - exerts the greatest influence on the timing of sexual maturation. Our findings indicate that the second trimester of pregnancy represents a particularly sensitive developmental window. Sunlight exposure during this period was strongly and inversely associated with age at menarche, even after adjustment for a broad range of confounders. Although sunlight exposure during the third trimester was also initially associated with pubertal timing, this relationship was attenuated and no longer statistically significant in fully adjusted models, suggesting a lesser or indirect role during late gestation. Findings consistent with our observations have been reported in studies of the UK Biobank, which examined the effects of birth season on birthweight, age at menarche, adult height, and body mass index (Day et al. 2015). In that large population-based analysis, estimated sunlight exposure during the second trimester of pregnancy emerged as a key determinant of pubertal timing in females. According to the authors, the absence of associations between sunlight exposure during other stages of the prenatal period and pubertal outcomes supports the hypothesis that this effect is programmed during foetal life.

Recent evidence furthermore highlights an essential association between sunshine exposure during the first trimester of pregnancy and the subsequent timing of pubertal development (Gaml-Sørensen et al. 2023). In the Danish Puberty Cohort - an extensive, population-based follow-up of children nested within the Danish National Birth Cohort - first trimester exposure occurring during November through April, a period characterized by low sun exposure and reduced estimated endogenous synthesis of vitamin D₃ in the skin, was associated with earlier attainment of multiple pubertal milestones in both girls and boys. Nevertheless, population-based observations such as these do not yet clarify the underlying physiological mechanisms, nor do they fully explain why associations between prenatal sunshine exposure and pubertal outcomes emerge only later in ontogeny.

The inverse association between age at menarche and average sunshine duration during the second trimester of pregnancy may be explained by the influence of vitamin D on critical events in female reproductive system development that occur during this prenatal period. During midgestation, approximately between the 20th and 24th weeks of pregnancy, peak levels of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) are observed (DiVall and Radovick 2008). This hormonal surge coincides with the formation and maturation of primordial ovarian follicles (Yao et al. 2021). Increased prenatal exposure to ultraviolet (UV) radiation may therefore influence the establishment of the hypothalamic–pituitary–ovarian (HPO) axis via vitamin D–mediated mechanisms, as vitamin D receptors are expressed in both the hypothalamus and the pituitary gland (Rodríguez-Rivero and Medina 2025).

Furthermore, vitamin D has been shown to upregulate aromatase expression (Dragomir et al. 2024), a key enzyme in steroidogenesis, which begins in the ovaries around the 20th week of gestation (Yao et al. 2021). Evidence from animal studies indicates that maternal vitamin D deficiency during pregnancy adversely affects reproductive physiology in female offspring by disrupting neuroendocrine regulation (Nichols et al. 2024), lending further support to the plausibility of this biological pathway. Overall, these findings suggest that greater maternal exposure to sunlight during the second trimester may have a beneficial effect on fetal reproductive development. Given that earlier age at menarche has been associated with favourable environmental conditions, the observed relationship may reflect a positive developmental effect of increased prenatal sunshine exposure.

Recent reports further indicate that maternal and neonatal vitamin D levels in the prenatal and perinatal periods may influence reproductive system function from the earliest stages of life. Specifically, vitamin D status around birth appears to affect the regulation of the HPG (hypothalamic–pituitary–gonadal) axis during infancy, particularly during the mini-puberty period. This early hormonal activation phase may have implications for future gonadal function and broader hormonal regulation across the life course (Kılınç et al. 2019).

In light of evidence indicating that even a balanced diet is insufficient to meet the vitamin D requirements of pregnant women (Iglesias-Vázquez et al. 2023), as well as the inconclusive findings from studies on vitamin D supplementation during pregnancy (Gaml-Sørensen et al. 2023). Further studies are needed to elucidate how vitamin D biology in early gestation, including its effects on endocrine pathways and developmental programming, affects the timing of sexual maturation.

The principal strengths of this study include its large sample size and the use of objective, month-specific data on sunshine duration throughout the prenatal period. In addition, the analyses incorporated a comprehensive set of covariates known to influence pubertal timing, including birthweight, birth order, and socioeconomic conditions. Nevertheless, several limitations should be acknowledged. Information on maternal diet, vitamin D status, physical activity, overall health during pregnancy, and time spent outdoors was unavailable. Furthermore, the use of national average sunshine duration rather than region-specific exposure data may have resulted in some degree of exposure misclassification. Data on individual sun exposure behaviours and vitamin D supplementation were also lacking. It is noteworthy, however, that the study participants were conceived prior to the introduction of official vitamin D supplementation guidelines in Poland, which may have minimized routine supplementation effects (Charzewska et al. 2010).

Conclusions

In conclusion, this study provides evidence of a significant association between prenatal sunlight exposure and the timing of menarche in girls, independent of season of birth. The second trimester of pregnancy appears to constitute a critical developmental window, during which higher levels of sunlight exposure are associated with earlier pubertal onset. These findings support the hypothesis that in utero vitamin D synthesis and related endocrine mechanisms contribute to the developmental programming of female sexual maturation.

Author contributions

Conceptualization and Methodology- M. K, W. U.; Data Collecting- M.K, W.U.; Statistical Analysis – M.K; Climatological Data Preparation - K. K.-K.; Original Draft Preparation – W. U. M.K.; Review and Editing – M.K., W.U., K. K.-K.

Funding

The authors did not receive support from any organization for the submitted work.

Data availability

The datasets generated and analysed during the current study are available from the corresponding author on reasonable request.

Declarations

Conflict of interest

The authors have no relevant financial or non-financial interests to disclose.

Ethical standards

The research protocol was reviewed and approved by the institutional ethics committee of the University of Wroclaw in accordance with applicable national regulations and with the 1964 Helsinki Declaration and its later amendments.

Consent to participate

Informed consent was obtained from all participants prior to their inclusion in the study.