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Early periods and changing fertility patterns may influence ovarian cancer risk - medicalxpress

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Ovarian cancer is still one of the deadliest gynecological cancers affecting women around the world, especially in East Asian countries, where the numbers keep rising year after year. A new nationwide study in South Korea analyzed more than 2 million women who were 40 years old or older to explore how reproductive experiences shape ovarian cancer risk in women born across four different decades, the 1930s to 1960s.

The findings published in JAMA Network Open show that having more children was linked to lower ovarian cancer risk, mainly among older generations, when larger families were more common. This protective effect was weaker or absent in more recent generations with lower fertility, suggesting that future generations may not experience the risk reduction previously associated with having multiple children.

The family size wasn't the only factor. The data revealed that other key aspects of reproductive history in women, such as their age at their first period, their menopausal status, and use of birth control, were also closely linked to ovarian cancer risk.

Survival plot of the incidence probabilities of ovarian cancer in premenopausal women. Credit: JAMA Network Open (2026). DOI: 10.1001/jamanetworkopen.2025.56840

Increasing burden of ovarian cancer

Ovarian cancer studies have suggested that earlier onset of menstruation slightly increases risk, while each additional childbirth substantially lowers it. However, most of this evidence comes from Western populations, where women have historically had more children and better access to oral contraceptives.

More recent studies, however, have pointed out that the protective effect of pregnancies isn't uniform. In countries where women have fewer children and have them later in life, the ovaries go through more ovulation cycles over a lifetime, which may increase cancer risk. At the same time, reproductive experiences have changed across decades. Differences in breastfeeding practices and birth control use have the potential to influence the risk of developing ovarian cancer.

South Korea has experienced one of the most rapid declines in fertility worldwide. The average number of children per woman fell from more than four in the 1970s to fewer than one in 2022. This stark demographic shift offers a rare and real-time view of how the loss of protection from multiple pregnancies may influence cancer patterns.

Furthermore, South Korea belongs to a region in East Asia where ovarian cancer is still on the rise, unlike many high-income countries around the world, where the number of cases has been falling. Despite this, there is a lack of robust demographic-specific study exploring the topic.

Identifying the risk trends

In this study, the researchers looked beyond the protective effect of pregnancy alone. The team examined how different aspects of reproductive history relate to ovarian cancer risk, and how these links vary depending on whether women are pre- or post-menopause and the generation they belong to.

They collected and analyzed data from the National Health Insurance Service (NHIS) database covering women in South Korea born across three different decades.

They found that starting periods at age 12 or younger and a longer reproductive lifespan were associated with a higher risk of ovarian cancer. Having two or more children generally reduced risk by about 30%, although this protective effect was less pronounced in younger women in the cohort (born in the 1960s).

Menopausal status also played an important role in shaping risk patterns. The team found that using birth control lowered ovarian cancer risk in pre-menopausal women but offered little to no protection for those who were post-menopausal. In addition, post-menopausal women were also at higher risk if they used hormone replacement therapy.

The researchers emphasized the importance of developing prevention strategies tailored to aging populations with low fertility. Understanding the trends highlighted in this study is an important step toward improving risk assessment and developing prevention strategies for women as the fertility landscape continues to evolve. 

by Sanjukta Mondal, Medical Xpress

edited by Lisa Lock, reviewed by Robert Egan 

Source: Early periods and changing fertility patterns may influence ovarian cancer risk