BACKGROUND Earlier onset of breast development (thelarche) is associated with increased breast cancer risk. Identifying modifiable factors associated with earlier thelarche may provide an opportunity for breast cancer risk reduction starting early in life, which could especially benefit girls with a greater absolute risk of breast cancer due to family history. METHODS We assessed associations of maternal pre-pregnancy body mass index (BMI), physical activity during pregnancy, gestational weight gain and daughters' weight and length at birth with age at thelarche using longitudinal Weibull models in 1031 girls in the Lessons in Epidemiology and Genetics of Adult Cancer from Youth (LEGACY) Girls Study-a prospective cohort of girls, half of whom have a breast cancer family history (BCFH). RESULTS Girls whose mothers had a pre-pregnancy BMI of ≥25 and gained ≥30 lbs were 57% more likely to experience earlier thelarche than girls whose mothers had a pre-pregnancy BMI of <25 and gained <30 lbs [hazard ratio (HR) = 1.57, 95% CI: 1.16, 2.12]. This association was not mediated by childhood BMI and was similar in girls with and without a BCFH (BCFH: HR = 1.41, 95% CI: 0.87, 2.27; No BCFH: HR = 1.62, 95% CI: 1.10, 2.40). Daughters of women who reported no recreational physical activity during pregnancy were more likely to experience earlier thelarche compared with daughters of physically active women. Birthweight and birth length were not associated with thelarche. CONCLUSION Earlier thelarche, a breast cancer risk factor, was associated with three potentially modifiable maternal risk factors-pre-pregnancy BMI, gestational weight gain and physical inactivity-in a cohort of girls enriched for BCFH.

中文翻译：

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LEGACY 女孩研究队列中的母亲和产前因素以及初生年龄：对乳腺癌风险的影响。

背景 乳房发育（乳房发育）较早开始与乳腺癌风险增加有关。确定与早期乳房发育相关的可改变因素可能会为从生命早期开始降低乳腺癌风险提供机会，这可能特别有益于因家族史而具有更大乳腺癌绝对风险的女孩。方法 我们在流行病学和遗传学课程中对 1031 名女孩使用纵向 Weibull 模型评估了母亲孕前体重指数 (BMI)、怀孕期间的身体活动、妊娠期体重增加和女儿出生时的体重和身长与年龄的关联青年成人癌症 (LEGACY) 女孩研究 - 一个前瞻性的女孩队列，其中一半有乳腺癌家族史 (BCFH)。结果 母亲孕前 BMI ≥ 25 且体重增加 ≥ 30 磅的女孩比母亲孕前 BMI < 25 且体重增加 < 30 磅的女孩更容易经历 57% [风险比 (HR ) = 1.57, 95% CI: 1.16, 2.12]。这种关联不受儿童 BMI 介导，并且在有和没有 BCFH 的女孩中相似（BCFH：HR = 1.41，95% CI：0.87，2.27；无 BCFH：HR = 1.62，95% CI：1.10，2.40）。与身体活跃的女性的女儿相比，在怀孕期间没有进行娱乐性体育活动的女性的女儿更有可能经历更早的乳房发育。出生体重和出生身长与乳房发育无关。结