Early thelarche, reproductive hormones, and MRI-based breast density: a prospective study in China

Abstract

Background

The age of thelarche in girls has declined globally. While earlier menarche has been linked to higher breast density - a known risk factor for breast cancer - the association between earlier thelarche and breast density remains unclear.

Methods

114 Girls presenting with idiopathic central precocious puberty without medication (ICPP), early thelarche (ET), and normal age at thelarche (NT) were recruited from pediatric endocrine clinics of three hospitals during the year of 2020–2021 in Hefei, Anhui Province, China and were followed up for 3–5 years. Participants had their breast composition measured by MRI at 1-year post menarche. Fasting blood samples were collected at baseline and follow-up to measure of reproductive hormone concentrations.

Results

The ET group and ICPP group exhibited significantly higher percent fibroglandular volume (%FGV: 42.1%, 52.0% ) and fibroglandular volume (FGV: 183.3 cm³, 237.0cm³) compared with the NT group (%FGV: 34.7%, FGV: 148.8cm³) . Serum estrogen levels at baseline and follow-up were elevated in the ET group relative to the NT group (52.1 vs. 40.7 pg/ml; 46.1 vs. 38.6 pg/ml), but did not differ significantly from the ICPP group (baseline: 54.6 vs. 52.1 pg/ml; follow-up: 48.9 vs. 46.1 pg/ml). Each 1-year earlier onset of thelarche was associated with a 7.8% increase in %FGV and a 35.7 cm³ increase in FGV. The combined hormonal pathway mediated the association between early puberty (age at thelarche < 9 years) and %FGV in this study, accounting for 28.7% (95% CI: 8.6% − 51.3%) of the total mediation effect. For the association between early puberty and FGV, baseline estradiol mediated 27.7% (95% CI: 11.8% − 44.8%), the pathway from baseline to follow-up estradiol mediated 13.6% (95% CI: 5.0% − 24.3%), and the combined hormonal pathway mediated 45.8% (95% CI: 18.1% − 68.0%).

Conclusion

In this cohort study, earlier thelarche (even between 8.0 and 9.0 years) was associated with higher %FGV and FGV. Prolonged estrogen exposure may represent a modifiable mediator that could be targeted for breast cancer prevention strategies in girls with earlier puberty.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13058-025-02116-4.

Background

Puberty represents a critical window of susceptibility for breast cancer, marked by rapid breast development that may heighten tissue vulnerablity to carcinogenic influences [1, 2]. Key pubertal milestones, such as thelarche (breast budding) and menarche, coincide with hormonal shifts and breast growth [3]. Globally, the timing of pubertal onset in girls has trended earlier in recent decades [4], raising concerns given that earlier menarche is a well-established risk factor for breast cancer [1, 5]. However, unlike menarche, fewer studies have examined the role of other pubertal milestones - particularly thelarche - in breast cancer risk. The timing of thelarche may hold greater biologically relevance, as it initiates the window of rapid breast proliferation. Supporting this, earlier thelarche was associated with 20–30% increased breast cancer risk in the UK Generations Study [6] and a US cohort of women with a family history of breast cancer [7].

However, potential misreporting of pubertal timing, particularly age at thelarche, may have obscured the association between thelarche and breast cancer risk in both premenopausal and postmenopausal women. Future studies employing prospective assessments of thelarche and menarche are needed to minimize measurement error and clarify the relationship between earlier pubertal onset and breast cancer risk.

An alternative approach to addressing this challenge is to evaluate intermediate markers earlier in life. Breast density—defined as the ratio of fibroglandular to adipose tissue --has been established as an intermediate marker for breast cancer and can be assessed in post-menarcheal adolescencets [8, 9]. Recent studies suggest that persistently dense or increasingly dense breasts are associated with 2-3-fold higher breast cancer risk compared to predominantly fatty breasts [10]. However, current evidence on the relationship between pubertal factors and mammographic density primarily comes from retrospective studies [11, 12]; only a few from prospective studies [9]. Epidemiological studies have reported inconsistent findings regarding the association between age at menarche and mammographic density. While some studies suggest that earlier menarche is linked to higher density [9, 13], a pooled analysis by Ward et al. found that later menarche was associated with higher mammographic density [14]. In contrast, it remains unclear whether girls with earlier breast onset–including precocious puberty (before age 8) or early thelarche (before age 9)--have an elevated risk of higher breast density during adolescence and young adulthood.

Substantial shifts in reproductive hormone levels at the onset of puberty drive mammary gland development, primarily through the actions of estrogen, IGF-1, and progesterone [15]. Exposure to exogenous estrogens during adolescence has been associated with increased breast density and a heightened risk of breast cancer [16]. One adolescent study reported that premenarchal levels of dehydroepiandrosterone sulfate (DHEAS), an adrenal steroid that can be metabolized into estrogens, are positively associated with breast density [17]. Thus, reproductive hormones may underlie the link between pubertal development and mammographic density.

To address these critical gaps in the literature, we conducted a prospective study of girls with precious puberty or early thelarche (before age 9), following them for breast density assessment using breast magnetic resonance imaging (MRI) one year post-menarche. Unlike mammography, breast MRI enables three-dimensional visualization of fibroglandular and adipose tissue components separately, with enhanced sensitivity to background parenchymal enhancement in fibroglandular tissue [18]. Our study also measured reproductive hormones concurrently with breast density assessments. This integrated approach may help uncover key mechanistic pathways linking early pubertal development to increased breast cancer susceptibility.

Methods

Study design and population

This study was conducted within an ongoing cohort study in Anhui Province, China. We recruited girls with untreated idiopathic central precocious puberty (ICPP), early thelarche (ET), and normal thelarche (NT) from pediatric endocrine clinics at three hospitals in Hefei between January 2020 and December 2021. Participants were followed up from May to November 2024. This study was approved by the Anhui Medical University (Approval No. 83230084).

All participants were children receiving regular growth and developmental assessments at the hospital since age 6. Those newly exhibiting thelarche between January 2020 and December 2021 (study baseline period) were invited to enroll. Consented participants underwent fasting blood sampling and baseline assessments on the subsequent day. Annual follow-up documented menarche status. At one year post-menarche, participants returned for standardized blood collection and mammographic density evaluation. ICPP inclusion criteria required: (1) pubertal onset before age 8 years, (2) biochemical evidence of hypothalamic-pituitary-gonadal (HPG) axis activation (random ultrasensitive LH > 0.3 mIU/ml or GnRHa stimulation test with peak LH > 5 mIU/ml), and (3) exclusion of organic causes via negative brain MRI. ET and NT groups were defined by Tanner stage II breast development occurring at 8.0–9.0 years and 9.0–10.0 years, respectively. Exclusion criteria for all groups included: (1) prior GnRHa treatment, (2) developmental delay, or (3) behavioral problems affecting interview participation.

Breast density measurement

Breast density was quantified using non-contrast magnetic resonance imaging (MRI) performed on a 1.5-Tesla or higher field-strength scanner equipped with dedicated breast radiofrequency coil [9]. A single investigator processed all MRI data by: (1) delineating the chest wall-breast tissue boundary and skin surface, and (2) segmenting fibroglandular and adipose tissue using ITK-SNAP software (version 4.2.0) [19]. This approach enabled measurement of total breast volume (BV), fibroglandular volume (FGV) and non-fibroglandular volume (NFGV). Percent fibroglandular volume (%FGV) was calculated the as (FGV/BV)×100. Density measures were averaged across both breasts for analysis. %FGV demonstrated strong correlation with mammographic breast density percentage (r = 0.87) [20], validating its use as our primary outcome measure. FGV served as a secondary outcome.

Pubertal assessment

Breast development was assessed by a female pediatric endocrinologist using Tanner staging [21]. Evaluations, including visual inspection and palpation, were conducted in a private room with the mother or another female guardian present. In cases where obesity precluded reliable assessment of breast bud presence, ultrasonography was used for adjunctive assessment. Age at thelarche was defined as the midpoint between the age at the visit when breast development was first observed to have reached Tanner stage B2 and the age at the previous examination. Participants subsequently received annual follow-ups to ascertain menarche status. The thelarche-to-menarche tempo was calculated as the age at menarche minus the age at thelarche.

Blood collection

Blood samples were collected at two time points: baseline (thelarche) and one-year post-menarche. Baseline samples were obtained via fasting venipuncture at 8:00 AM on the morning following enrollment. For one year post-menarche sampling, participants were required to present during menstrual cycle days 2–5, during which fasting blood collection was performed at 8:00 AM under standardized conditions.

Covariates

Covariates were obtained through: (1) questionnaire data: self-reported and mother-reported information from baseline and follow-up surveys; (2) Clinical measurement: anthropometrics (weight, height, waist-to-hip ratio) were assessed by pediatricians at baseline and 1-year post-menarche. Body fat percentage was measured via bioimpedance; (3) Family history data: parental education, maternal reproductive history, maternal menstrual history, and breast cancer family history were collected through maternal questionnaires.

Statistical analysis

For demographic characterization, continuous variables are presented as mean ± standard deviation (SD), while categorical variables are reported as frequencies (percentages). Continuous variables were compared using one-way ANOVA with Tukey’s post hoc tests, while categorical variables were analyzed with chi-square tests for between-group comparisons. Multiple linear regression models were employed to analyze the relationship between age at thelarche and breast density both as continuous variables. Mixed-effect regression models were employed to analyze the relationship between thelarche groups and breast density. We constructed three models Model 1 was not adjusted for any covariates. Model 2 adjusted for whole-body percent fat measured by bioimpedance device (continuous), and age at menarche (continuous). Because childhood BMI is a strong predictor of pubertal timing and has been consistently associated with breast density, Model 3 adjusts for the covariates in Model 2, with additional adjustment for thelarche-to-menarche tempo, baseline BMI z-score, maternal education, paternal education, and family history of breast cancer.

Structural equation modeling (SEM) is a comprehensive statistical approach that allows the estimation of complex relationships among observed and latent variables. In this study, SEM was applied to evaluate the hypothetical pathways from early puberty (age at thelarche < 9.0 years) through reproductive hormone levels to %FGV and FGV. Birth weight, age at menarche, breast cancer family history, body fat percentage at follow-up and BMI at baseline were included as confounders in the model. The hypothesized model was developed based on prior biological knowledge and presumed temporal sequence of endocrine changes (Figure S1). SEM was estimated using the robust maximum likelihood method, which performs well in the presence of moderate non-normality. Model fit was evaluated using the root mean square error of approximation (RMSEA), standardized root mean square residual (SRMR), comparative fit index (CFI), and Tucker–Lewis index (TLI). Indirect effects along specific mediating pathways were quantified using the product-of-coefficients approach, and mediation proportions were calculated to reflect the extent to which each pathway contributed to the overall association between early puberty and FGV and %FGV. All analyses were conducted using STATA 13.0 (College Station, TX) and R software (version 4.3.1; R Core Team, 2023) with the “lavaan” package.

Results

Between January 2020 and December 2021, we enrolled 137 eligible girls in the study cohort, with biospecimens collected from 95 participants. Of these, 114 participants (83.2%) completed follow-up assessments after a mean of 3.6 years (range: 2.8–4.9 years), including breast MRI examinations and paired biological sample collection (Figure S2).

At follow-up, mean ages differed significantly among groups: 11.1 ± 0.7 years for the ICPP group (n = 29), 12.0 ± 0.8 years for the ET group, and 12.5 ± 0.8 years for the NT group. However, the groups showed no significant differences in BMI, bone age, body fat percentage, waist-to-hip ratio, or demographic characteristics (Table 1).

Table 1.

Characteristics and outcomes of the total study population

	All	ICPP	ET	NT	P-valuea ET: ICPP/ NT
	N = 114	N = 29	N = 48	N = 37
Participants characteristics at baseline
Age, mean (± SD), y	8.9 (0.9)	7.7 (0.5)	8.9 (0.4)	9.9 (0.4)	< 0.001/ <0.001
BMI z-score	0.03 (1.3)	0.2 (1.3)	0.1 (1.4)	-0.1 (1.1)	0.72/ 0.46
Bone age, mean (± SD), y	9.8 (0.9)	9.6 (0.8)	9.8 (0.8)	9.9 (1.1)	0.36/ 0.50
BA-CA, mean (± SD), y	1.0 (0.8)	1.8 (0.6)	1.0 (0.7)	0.4 (0.7)	< 0.001/ <0.001
Participants characteristics at follow-up
Age (y), mean (± SD), y	12.1 (0.9)	11.1 (0.7)	12.1 (0.7)	12.9 (0.6)	< 0.001/ 0.001
BMI z-score	0.4 (1.1)	0.6 (1.3)	0.5 (0.8)	0.2 (1.1)	0.70/ 0.18
Bone age, mean (± SD), y	13.5 (1.0)	13.2 (1.2)	13.6 (1.0)	13.7 (0.8)	0.09/ 0.69
Age at menarche, mean (± SD), y	10.9 (0.8)	10.2 (0.6)	11.0 (0.7)	11.5 (0.7)	< 0.001/ 0.001
Body fat percentage, mean (± SD), %	23.6 (7.2)	25.1 (7.5)	24.3 (6.2)	21.5 (7.9)	0.63/ 0.08
Family characteristics
Maternal education					0.72/ 0.49
≤ High school	35 (30.7)	24.1	29.2	37.8
College degree	43.0	44.8	47.9	35.1
Graduate school	26.3	31.1	22.9	27.1
Paternal education					0.66/ 0.79
≤ High school	28.9	27.6	27.1	32.4
College degree	42.1	48.3	39.6	40.5
Graduate school	28.9	24.1	33.3	27.1
Maternal height, mean (± SD), cm	158.8 (4.4)	159.5 (4.2)	159.2 (4.5)	157.6 (4.4)	0.84 / 0.17
Paternal height, mean (± SD), cm	172.5 (5.7)	173.4 (7.0)	172.3 (5.5)	172.2 (4.8)	0.50 / 0.94
Family history of breast cancer					0.11/ 0.16
Yes	8.8	3.4	14.7	5.4
No	91.2	96.6	83.3	94.6
Breast composition parameters
%FGV, %	42.3 (13.7)	52.0 (11.8)	42.1 (13.0)	34.7 (10.9)	0.001** / 0.006**
FGV, mean (± SD), cm3	185.8 (77.9)	237.0 (84.6)	183.3 (65.2)	148.8 (66.5)	0.002** / 0.03*
NFGV, mean (± SD), cm3	272.7 (138.2)	224.8 (95.5)	265.9 (110.7)	320.0 (180.5)	0.2 / 0.06
BV, mean (± SD), cm3	460.2 (168.2)	461.8 (153.5)	448.4 (135.6)	474.2 (214.6)	0.7 / 0.5
Reproductive hormones
Estradiol at baseline (pg/ml)	50.0 (22.7)	54.6 (24.5)	52.1 (20.2)	40.7 (22.9)	0.6/ 0.048*
Missing, n	19	0	6	13
Estradiol at follow-up, mean (± SD), pg/ml	44.4 (17.5)	48.9 (24.5)	46.1 (16.8)	38.6 (15.9)	0.5/ 0.047*
Progesterone at follow-up, mean (± SD), ng/ml	0.8 (1.8)	0.8 (1.7)	1.0 (2.2)	0.6 (0.9)	0.6/ 0.3
IGF-1 at follow-up, mean (± SD), ng/ml	334.5 (72.4)	328.8 (93.9)	339.6 (60.1)	332.3 (69.2)	0.5/ 0.7

^a: First P-value (ET vs. ICPP), second P-value (ET vs. NT)

Note : BMI z-score represents the body mass index standardized for age and sex. ICPP, idiopathic central precocious puberty; ET, early thelarche; NT, normal thelarche; BA, bone age; CA, chronological age; BMI, body mass index; FGV, fibroglandular volume; %FGV, percent of fibroglandular volume; NFGV, non fibroglandular volume; BV, breast volume; LH, luteinizing hormone; FSH, follicle-stimulating hormone; IGF-1, insulin-like growth factor 1; SD, standard deviation

Breast composition analysis

As shown in Fig. 1, significant differences emerged among the three groups for all breast composition parameters (FGV, %FGV, NFGV) except breast volume. Both the ET and ICPP groups demonstrated significantly higher %FGV compared to the NT group (Fig. 1D ET: 42.1% vs. 34.7%, P = 0.006; ICPP: 52.0% vs. 34.7%, P < 0.001). Similarly, FGV values were elevated in both early-development groups (Fig. 1A ET: 183.3 cm³ vs. NT: 148.8 cm³, P = 0.03; ICPP: 237.0 cm³ vs. NT: 148.8 cm³, P = 0.002).

Fig. 1.

Description of breast composition in girls at different groups. The figure represents the mean and standard deviation of breast composition components (A, FGV; B, NFGV; C, BV; and D, %FGV) of this study: ICPP: idiopathic central precocious puberty; ET: early thelarche; NT: normal thelarche; FGV: fibroglandular volume; %FGV: % of fibroglandular volume; NFGV: non fibroglandular volume; BV: breast volume

Hormonal profile observations

Notably, despite these differences in breast composition, baseline (54.6 vs. 52.1 pg/mL, P > 0.05) and follow-up (48.9 vs. 46.1 pg/mL, P > 0.05) estrogen levels were comparable between the ICPP and ET groups. Both groups maintained significantly higher estrogen concentrations than the NT group at all time points (Table 1).

Association between thelarche timing and breast density

As presented in Table 2, fully adjusted models (Model 3) revealed a significant inverse relationship between age at thelarche and both FGV (P_trend < 0.001) and %FGV at 1 year post-menarche. Compared to the NT group [EMM: 150.7 cm³ (95% CI: 117.6-183.8)], the ET group demonstrated significantly higher FGV [EMM: 183.1 cm³ (95% CI: 157.2-209.1)]. Notably, each one-year earlier onset of thelarche was associated with a 35.7 cm³ increase in FGV and a 7.8% greater %FGV. In contrast, testosterone levels, both at baseline and one year after menarche, were not significantly associated with either FGV or %FGV (Table S1).

Table 2.

Adjusted estimated marginal means by thelarche group

Outcomes	Thelarche groups	Estimated marginal means (95%CI)
		Model 1	Model 2	Model 3
FGV	ICPP	237.0 (210.9, 263.2)	233.3 (203.9, 262.6)	223.5 (168.7, 278.3)
	ET	183.3 (162.9, 203.6)	180.5 (160.6, 200.3)	176.2 (150.1, 212.5)
	NT	148.8 (125.7, 171.9)	154.4 (129.3, 179.4)	145.1 (115.8, 196.1)
	P trend	< 0.001	< 0.001	< 0.001
%FGV	ICPP	52.1 (47.6, 56.5)	50.9 (45.9, 56.0)	51.5 (40.2, 62.8)
	ET	42.1 (38.7, 45.6)	42.5 (39.1, 45.9)	41.4 (39.7, 56.9)
	NT	34.7 (30.8, 38.7)	35.1 (30.8, 39.4)	36.6 (30.1, 42.1)
	P trend	< 0.001	< 0.001	< 0.001

Estimated marginal means from mixed-effect regression models for the thelarche group in relation to breast MRI factors

Model 1 is unadjusted;

Model 2 adjusts for the following variables as fixed effects: whole-body fat percemtage (%, continuous) and age at menarche (continuous). Clinic included as a random effect;

Model 3 adjusts for the same factors in model 2, and in addition, thelarche-to-menarche tempo (continuous), BMI at baseline expressed as a z-score (continuous), Maternal education (≤ high school (reference), college degree, graduate school), paternal education (≤ high school (reference), college degree, graduate school), family history of breast cancer (yes vs. no)

Mediation analysis of early puberty and breast density

Figure 2 and Table S2 present the structural equation model examining reproductive hormones as mediators between early puberty (ET and ICPP groups) and %FGV. The model demonstrated good fit [χ² = 9.3, P = 0.09; CFI = 0.9; TLI = 0.9; SRMR = 0.07; RMSEA = 0.09 (90% CI: 0.2, 0.7)], supporting our hypothesized pathways. Significant associations were found between early puberty with baseline estradiol and follow-up estradiol with %FGV (β = -0.8, P = 0.04). Non-significant associations were found between early puberty with IGF-1 (β = -0.2, P = 0.6) and progesterone (β = 0.03, P = 0.8), and between baseline estradiol with %FGV (β= -0.8, P = 0.2). While individual hormone - %FGV paths were non-significant (all Ps > 0.05), the combined mediation effect was substantial (28.7%).

Fig. 2.

Mediation of the relationship between early puberty and %FGV. The figure represents the mediating roles of baseline/ follow-up estradiol (E2) and follow-up IGF-1/progesterone in the association between early puberty and %FGV. Note: Path coefffcients were standardized. ^a Early puberty: included ICPP group and ET group. *p < 0.05, **p < 0.01, ***p < 0.001

Structural equation modeling results for FGV

The proposed model demonstrated good fit to the data (χ² = 9.3, df = 5, P = 0.09; CFI = 0.98; TLI = 0.93; SRMR = 0.07; RMSEA = 0.09 [90% CI: 0.001–0.19]), as shown in Fig. 3 and Table S3. Key path coefficients revealed significant associations between early puberty with baseline estradiol (β = -0.3, P = 0.002), and FGV (β = 0.4, P < 0.001). Strong predictive relationships were observed with baseline estradiol and follow-up estradiol (β = -5.6, P = 0.03) to FGV. The significant direct path from early puberty to FGV, along with a substantial mediation effect (45.8%), indicates that baseline and follow-up estradiol partially mediate this association, with additional contributions from IGF-1 and progesterone at follow-up.

Fig. 3.

Mediation of the relationship between early puberty and FGV. The figure represents the mediating roles of baseline/ follow-up estradiol (E2) and follow-up IGF-1/progesterone in the association between early puberty and FGV. Note: Path coefffcients were standardized. ^a Early puberty: included ICPP group and ET group. *p < 0.05, **p < 0.01, ***p < 0.001

Stratified analysis of FGV by thelarche group and estradiol levels

Table 3 presents the mixed-effects regression results stratified by thelarche group. Girls with early puberty and high baseline estradiol showed significantly greater FGV compared to those with normal thelarche and low baseline estradiol (EMM = 124.3 cm³, 95% CI: 93.2-155.5; P < 0.001). After adjustment for baseline estradiol level (using high follow-up estradiol with early puberty as reference), the normal thelarche group with high follow-up estradiol maintained significantly lower FGV values (EMM difference=-63.1 cm³, 95% CI: -97.5 to -28.7; P < 0.001), as detailed in Table S4. As a sensitivity analysis, we stratified participants by thelarche-to-menarche tempo. After adjusting for estrogen levels at either baseline or follow-up, FGV remained significantly lower in the group with low estrogen levels and short tempo compared to those with high estrogen levels and long tempo (Table S5).

Table 3.

Adjusted FGV estimated marginal means by thelarche group and Estrogen exposure

	Number	Mean (95% confidence interval)	P - value
E2 at baseline as fixed effect
Low E2 at follow-up/ NT	21	77.1 (31.7, 122.6)	Reference
Low E2 at follow-up/ EP	37	13.7 (-24.9, 52.4)	0.15
High E2 at follow-up/ NT	16	30.8 (-11.3, 72.9)	0.48
High E2 at follow-up/ EP	40	93.9 (57.8, 130.1)	<0.001
E2 at follow-up as fixed effect
Low E2 at baseline/ NT	18	72.8 (33.9, 111.8)	Reference
Low E2 at baseline/ EP	30	27.7 (-3.5, 58.9)	0.08
High E2 at baseline/ NT	19	68.2 (30.8, 105.6)	<0.001
High E2 at baseline/ EP	47	124.3 (93.2, 155.4)	<0.001

Estimated marginal means from mixed-effect regression models, stratified by thelarche group, median of E2 at baseline, and E2 at follow-up in relation to fibroglandular volume (FGV)

Note: Adjusts for whole-body fat percemtage (%, continuous), age at menarche (continuous), thelarche to menarche tempo (continuous), BMI at baseline expressed as a z-score (continuous), Maternal education (≤ high school (reference), college degree, graduate school), paternal education (≤ high school (reference), college degree, graduate school), family history of breast cancer (yes vs. no)

Discussion

This is the first prospective study in girls to examine the association between earlier thelarche (onset before age 9) and MRI-based breast density—a well-established breast cancer risk factor—while also exploring the mediating role of reproductive hormone levels. Our findings show that each one-year earlier onset of thelarche was associated with a 35.7 cm³ increase in fibroglandular volume (FGV) and a 7.8% increase in percent FGV (%FGV). Notably, girls who experienced thelarche before age 8, as well as those between 8.0 and 9.0 years, exhibited significantly higher %FGV and FGV one year post-menarche compared to those with thelarche onset after age 9. These findings indicate that earlier thelarche, even within the clinically normal range and without meeting criteria for precocious puberty, is linked to substantially greater breast density.

Both baseline and follow-up estrogen levels mediated the association between earlier thelarche and increased breast density, with baseline levels showing a stronger mediating effect. In stratified analyses restricted to participants with consistently elevated estradiol levels (either at baseline or follow-up), girls with thelarche before age 9 had significantly higher FGV values than those with later thelarche onset.

Compared with the GOCS cohort, our study reported generally similar FGV, %FGV, and BV values [8]. However, as nearly one-third of the children in our cohort had ICPP, the values observed in participants with comparable ages at breast development were consequently lower than those reported in the GOCS cohort. The use of MRI in our study—recognized for its superior accuracy in assessing breast tissue composition—instead of DXA as used in GOCS, may account for some differences [22, 23]. Moreover, the higher prevalence of childhood obesity in the Western population [24], compared to Chinese children, may also contribute, given the influence of adiposity on breast development and volume.

The relationship between early thelarche and breast density remains insufficiently characterized. To date, only one prospective study—the Dietary Intervention Study in Children (DISC)—has examined this association, reporting a significant link between early thelarche and adult breast density after adjusting for BMI [9]. However, the findings from this study may not accurately reflect the effects of truly early thelarche, as the DISC cohort had a mean thelarche age of 10.6 years, with fewer than 5% of participants experiencing onset before age 9. More critically, DISC measured breast density in adulthood (mean age 27.2 years), by which time nearly one-third of participants had undergone pregnancy. Given pregnancy’s known transformative effects on breast tissue [15], adult assessments may mask thelarche’s direct biological influence.

Our prospective adolescent cohort study addresses these limitations by evaluating breast development during its dynamic, pre-pregnancy phase. Notably, over half of the participants in our cohort experienced thelarche before age 9, allowing for more robust assessment of early onset. The strong associations we observed suggest that previous research may have systematically underestimated the impact of early thelarche on breast density.

In our study, estrogen levels were positively associated with breast density, a finding consistent with several reports in perimenopausal women [25–27]. However, other studies have reported either inverse or null associations between estrogen and breast density in middle-aged and older women [28, 29]. Among adolescents, exogenous estrogen exposure between ages 12 and 17 has been linked to higher adult breast density [16]. Elevated IGF-1 levels during adolescence have also been associated with increased breast cancer risk [30]. In contrast, studies in young adult women have found no significant association between circulating estrogen and breast density [31]. These inconsistencies may reflect the dynamic and stage-dependent effects of reproductive hormones on breast tissue across the female lifespan. In supplementary analyses, we examined the association between circulating androgens and breast density; these analyses did not reveal a significant relationship. This lack of association may reflect an age-specific effect of androgens, whose influence on breast cancer risk has been primarily documented in adults and may not extend to children [32–34]. Further studies are warranted to investigate this potential relationship more comprehensively across different developmental stages. Therefore, interpreting the relationship between reproductive hormones and breast density requires careful consideration of population characteristics and developmental timing. As current evidence is largely derived from adult populations, further studies in adolescent cohorts are needed to clarify these associations.

Although prior research has proved the complex association between reproductive hormones and breast density, their role in the association between early puberty and breast density remains unexamined. While estradiol, IGF-1, and progesterone are well-established regulators of pubertal mammary development [15], their role in mediating the relationship between earlier thelarche (< 9 years) and breast density had not been previously investigated. Our mediation analysis demonstrates that these hormones collectively account for 28.7% of this association, with estradiol exhibiting the most pronounced mediating effect. This hierarchical pattern—where estrogen’s influence surpasses that of IGF-1 and progesterone—aligns with existing models of estrogen receptor-driven mammary proliferation [35, 36]. These findings suggest that earlier thelarche may trigger enduring morphological changes through endocrine programming, potentially establishing lifelong trajectories of breast cancer risk.

In recent years, the incidence of breast cancer has shown a steady increase with a concerning trend toward younger age at diagnosis [37, 38], underscoring the critical need to identify early risk factors and explore potential preventive interventions. Prior epidemiological studies have established an association between earlier age at menarche and increased breast cancer risk; since early thelarche typically precedes and contributes to early menarche, the onset age of breast development may represent a more proximal and biologically significant exposure in breast carcinogenesis. Evidence from retrospective studies, including the UK Breakthrough Generations Study and the US Sister Study, suggests that earlier thelarche is associated with a 20–30% increased risk of breast cancer later in life [6, 7]. The secular trend toward earlier thelarche - advancing at 0.25 years/decade worldwide [4] - exhibits disturbing synchrony with two key epidemiological shifts: rising breast cancer incidence and younger age at diagnosis. This temporal synchrony provides compelling evidence that pubertal timing plays a causative role in breast carcinogenesis, establishing thelarche not only as a biological milestone but also as a promising target for preventive interventions.

Our findings demonstrate that estrogen levels during thelarche onset serve as a key mediator linking earlier puberty to increased breast density. This observation aligns with established evidence from middle-aged and older women, where elevated circulating estrogen concentrations are positively associated with breast cancer incidence [39, 40]. These results provide empirical support for the mechanistic hypothesis that earlier estrogen exposure during puberty induces persistent morphological changes in breast tissue, potentially increasing lifetime breast cancer risk [6]. The clinical implications of these findings are that they identify peripubertal estrogen exposure as a modifiable factor influencing breast density. These insights may inform future strategies for breast cancer risk reduction, particularly in populations experiencing earlier pubertal timing.

While age at menarche is routinely documented in epidemiological studies, thelarche timing remains significantly underreported. Future research should prioritize standardized assessment of thelarche onset to better elucidate its independent association with breast cancer risk and evaluate its potential for improving existing risk prediction models. The incorporation of this metric into clinical practice could facilitate earlier identification of high-risk individuals who might benefit from personalized screening approaches or targeted preventive strategies. Although additional longitudinal investigations are required to establish causal relationships, our findings provide compelling biological evidence supporting exploration of puberty-delaying interventions in high-risk populations (particularly girls with familial breast cancer predisposition) and highlight the clinical value of incorporating endocrine marker monitoring into routine adolescent health evaluations.

This study has several notable strengths. To our knowledge, it represents the first longitudinal investigation examining the association between early thelarche and breast density in adolescents, featuring serial hormonal profile assessments at two critical developmental time points. The study’s limitations include a relatively small sample size which, while sufficient for initial hypothesis testing, may constrain statistical power for subgroup analyses and limit generalizability to broader populations. However, this limitation is partially mitigated by the deliberate inclusion of participants spanning the full continuum of normal and precocious puberty, ensuring adequate phenotypic variability.

Another limitation is that breast density assessments were conducted at an average age of 12 years, when breast development was ongoing rather than complete. At the time of imaging, the majority of participants had reached Tanner stage 5, but not all, indicating some variability in developmental status. Although this precludes definitive conclusions about final adult breast density patterns, the ongoing follow-up of this cohort will enable future evaluations at later developmental stages, providing valuable insights into long-term density trajectories in girls with early puberty. Finally, as with all observational studies, residual confounding by unmeasured environmental or genetic factors cannot be entirely ruled out. Nevertheless, the consistency of our findings with established biological mechanisms and prior epidemiological evidence lends credibility to our conclusions. Future studies with larger samples, more frequent assessments, and extended follow-up into adulthood will be essential to validate and expand upon these important preliminary findings.

Conclusions

Our study demonstrates that earlier thelarche onset - even in the absence of a precocious puberty diagnosis - is significantly associated with increased fibroglandular breast volume (FGV) and percentage fibroglandular volume (%FGV). Notably, estrogen levels during the initial phases of breast development emerged as a critical mediator in this relationship. These findings have important implications for both early-onset breast cancer prevention strategies and public health policy formulation. Future research directions should include longitudinal monitoring of girls with earlier thelarche in this cohort to characterize their breast density progression patterns throughout development.

Supplementary Information

Below is the link to the electronic supplementary material.

Author contributions

Ying Sun: Writing - review & editing, Supervision, Funding acquisition, Conceptualization. Wenqing Liu: Investigation, Data curation. Ying Li: Writing - review & editing, Writing - original draft, Investigation, Conceptualization. Yue Yu: Investigation, Data curation. Ya Wang: Investigation, Data curation. Deyun Liu: Supervision, Conceptualization, Funding acquisition. Liwei Zou: Methodology, Formal analysis.

Funding

This work was supported by the National Natural Science Foundation of China (No. 82173537, 82373591), the Key Scientific Research Project of Anhui Provincial Universities (No. 2024AH040105), and the Enhancement Program for Basic and Clinical Cooperative Research of Anhui Medical University (No. 2022xkjT035).

Data availability

The data that support the findings of this study are available from the corresponding author, YS, upon reasonable request.

Declarations

Ethics approval and consent to participate

This study was approved by the Anhui Medical University (Approval No. 83230084). Written informed consent was obtained from all mothers, with additional assent collected from girls aged ≥ 7 years.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.