These two factors, pregnancy (parity) and breastfeeding, generally prevent ovulation and may slow the natural depletion of ovarian follicles over time, the authors point out. Thus pregnancy and breastfeeding are believed to lower the risk of early menopause, which is defined as the end of menstruation before age 45. Early menopause is associated with increased risk of cardiovascular disease, cognitive decline and osteoporosis. Details appear in the online open-access journal JAMA Network Open.

Langton and colleagues suggest that while their results did not demonstrate a clear dose-response for breastfeeding, their findings are consistent with the hypothesis that a biological mechanism is influenced by longer breastfeeding. Also, findings from other studies support that optimal benefits from breastfeeding exclusively, for both mother and baby, reach a threshold between six and 12 months. Breastfeeding exclusively means that the baby receives no liquids or solid foods, only breast milk.

As Langton explains, “In our study, women with three or more pregnancies who breastfed exclusively for a total of 7 to 12 months had about a 32% lower risk of early menopause compared to women with the same number of children who breastfed exclusively for less than one month.” Langton and colleagues also observed what they call other notable associations. Compared to women with no full-term pregnancies, women who had two pregnancies had a 16% lower risk of early menopause and women with three pregnancies saw a 22% lower risk.

The authors believe theirs is the first prospective cohort study to look at the relationship between breastfeeding and risk of early menopause. They point out that only one other study had examined the relationship of parity (number of children) to risk of early natural menopause.

Langton says, “Our study has a lot of strength because of the large sample size, the 26 years of follow-up and the prospective design. Also, at baseline we limited our study to women who were premenopausal, which is a key point,” she adds, along with their careful control for many confounders.

“Our breastfeeding findings not only add new insight into ways to prevent early menopause,” she points out, “but they align nicely with recommendations of both the American Academy of Pediatrics and World Health Organization that US women exclusively feed their infants breastmilk for at least six months and continue breastfeeding for up to one year,” Langton notes.

For this work, Langton and colleagues conducted their prospective, population-based study within the on-going Nurses’ Health Study II cohort that began collecting data in 1989. They report that response rates were 85-90%. Parity, menopause status and age were measured at baseline and every two years, while breastfeeding factors were assessed three times during follow-up. Women in the study were followed until menopause or age 45, hysterectomy, oophorectomy, death, cancer diagnosis or loss to follow-up.