Breastfed girls may start puberty later
A study of 4,745 children found girls fed human breast milk, especially for four months or more without other foods, showed less advanced puberty at age 10.
Girls who consumed human breast milk as infants tended to start puberty slightly later than those fed only with infant formula, according to a large study published in The Journal of Nutrition. The research, which tracked 4,745 children from birth, suggests early feeding practices might influence the timeline of sexual development in females, though the same pattern was not observed in boys. You can explore more about health and development stats.
Over recent decades, medical professionals have noted a trend of puberty starting at younger ages, particularly in girls. Early puberty is a public health concern because it is linked to a higher risk of adult health issues like type 2 diabetes, cardiovascular disease, and certain reproductive cancers.
Study Design and Methods
Led by researcher Marta Pinto da Costa at the University of Porto in Portugal, the team used data from the Generation XXI birth cohort. This project enrolled expectant mothers at public maternity hospitals in Porto between 2005 and 2006. Parents reported infant feeding practices when their children were six, 15, and 24 months old, and again at four years old.
The children were categorized into four groups based on their early diets.
| Infant Feeding Category | Description |
|---|---|
| Never received human milk | Fed only with infant formula or other foods from birth. |
| Received human milk (mixed) | Fed human milk along with formula or other foods. |
| Exclusive human milk (<4 months) | Fed only human milk for less than four months. |
| Exclusive human milk (≥4 months) | Fed only human milk for four months or longer. |
The four-month cutoff was significant because Portugal provides four months of fully paid maternity leave, which heavily influences feeding timelines. When the children reached age 10, trained nurses evaluated physical development using markers like breast growth and pubic hair, grading them on the Tanner stage scale. The researchers combined these into a total puberty score, where a score of three or higher indicated puberty had started. For more on tracking developmental milestones, see our injuries and health data.
Key Findings for Girls
The analysis revealed a clear pattern for girls. Those who received any human milk had a lower likelihood of showing advanced puberty scores at age 10 compared to girls never fed human milk. The effect was strongest for girls fed only human milk for at least four months.
The initial data also indicated that exclusive human milk feeding was linked with a later onset of menarche, or the first menstrual cycle. However, this specific link lost statistical significance after the researchers accounted for the girls' body mass index at age 10. This suggests childhood body weight might partly explain the relationship between early feeding and the timing of menstruation.
Researchers speculate on a potential mechanism. "Human milk contains hormones like leptin, which helps regulate infant appetite," the source notes. While leptin influences the pathways that trigger reproductive maturity, how it from breast milk shapes long-term hormone levels remains unclear. Breast milk may indirectly delay puberty by helping prevent rapid weight gain in early childhood.
Results for Boys and Study Limitations
In boys, the researchers did not find consistent statistical links between human milk feeding and puberty scores. One group-boys who received some human milk but were never fed only it-showed a lower chance of having the highest puberty scores. The researchers cautioned this finding was based on very few boys who had reached advanced puberty by age 10, making it less reliable. Boys generally enter puberty later than girls, making early signs harder to detect at age 10.
The study is observational and cannot prove human milk directly causes a delay. Other factors associated with families who breastfeed, like higher maternal education or healthier lifestyle habits, might contribute. Infant feeding details were also reported by parents retrospectively, which could introduce memory errors.
Future research could track hormone levels to see if compounds in human milk interact with a child's endocrine system over time. Evaluating boys at older ages would also provide a more complete picture of their maturation. The study offers a new perspective on how early relationship dynamics, like feeding, can have long-term developmental consequences.
