A large cohort study found significant associations between sibling relatedness and earlier pubertal development in both girls and boys. Children with half/step or no siblings entered puberty earlier than those with only full siblings, suggesting that family structure plays a role in pubertal timing.
The study, published in Annals of Epidemiology, analyzed data from 10,657 children (5579 girls and 5078 boys) participating in the Danish National Birth Cohort's Puberty Cohort.
Key findings revealed:
- Girls with both full and half/step siblings entered puberty 1.2 months earlier (95% CI, -2.5 to 0.1 months) than those with only full siblings.
- Girls with only half/step siblings entered puberty 2.2 months earlier (95% CI, -3.7 to -0.7 months).
- Girls without siblings entered puberty 5.5 months earlier (95% CI, -8.5 to -2.5 months).
- Boys with both full and half/step siblings entered puberty 1.4 months earlier (95% CI, -2.7 to -0.1 months) than those with only full siblings.
- Boys with only half/step siblings entered puberty 1.2 months earlier (95% CI, -3.1 to 0.6 months).
- Boys without siblings entered puberty 4.5 months earlier (95% CI, -8.8 to -0.3 months).
The study utilized interval-censored regression models to estimate adjusted mean age differences in attaining various pubertal markers, accounting for several confounding factors, including maternal age at menarche, maternal BMI, planned pregnancy, maternal social status, maternal smoking and alcohol consumption during pregnancy, parental cohabitation status, maternal age at delivery, and birth weight.
Sample characteristics showed that:
- 73% of children had only full siblings
- 15% had both full and half/step siblings
- 10% had only half/step siblings
- 1% had no siblings
Children with half/step siblings or no siblings were more likely to have non-cohabiting parents, unplanned pregnancies, and mothers with higher smoking rates and lower social status. These children also had lower birth weights and higher internalizing and externalizing symptoms.
Pubertal markers assessed included:
- Girls: Tanner stages 1-5 for breast and pubic hair development, age at menarche.
- Boys: Tanner stages 1-5 for genital and pubic hair development, voice break, first ejaculation.
- Both sexes: Axillary hair and acne.
Data were collected through self-reported web-based questionnaires, completed biannually from age 11 until full maturity or age 18. STATA's -intreg-package was used for statistical analysis, with Huber-White robust variance estimation. Inverse probability weights addressed sampling approach and potential selection bias.
Maternal and childhood factors included:
- Mean maternal age at delivery: 30.6-31.4 years across sibling groups.
- Mean birth weight: 3356-3559 grams across sibling groups.
- Mean SDQ (Strengths and Difficulties Questionnaire) score: 13.9-16.2 across sibling groups.
- Mean childhood BMI: 15.6-15.7 kg/m² across sibling groups.
The study achieved a 70% participation rate, with 15,819 children returning at least one measurement of pubertal development. Multiple sub-analyses, such as stratification by parental cohabitation status and adjustment for childhood internalizing and externalizing symptoms and BMI, yielded results similar to the main findings.
Limitations included the potential misclassification of sibling relatedness due to self-reporting and the inability to determine the timing or changes in sibling relatedness over time. Additionally, the study's sample included fewer only children (1%) compared to the general Danish population (5%).
Several authors report financial support from entities including the European Research Council, Independent Research Fund Denmark, and the Research Council of Norway. The funding sources were not involved in the analysis, interpretation of data, or the submission of the report.