Women with a pre-eclampsia history had lower breast cancer risk, Finnish data show

Pre-eclampsia, a pregnancy complication involving high blood pressure and organ strain, has been separately linked to altered hormone exposure during pregnancy, and prior research has raised the possibility it affects a woman's long-term breast cancer risk. A study published this month in the Journal of Clinical Endocrinology & Metabolism tests that link at national scale in Finland, and adds a specific, clinically relevant question: does a history of pre-eclampsia change how hormone therapy later affects breast cancer risk [s1].

The design

Researchers conducted a register-based cohort study using the Finnish Cancer Register (1987–2023) for breast cancer outcomes and the National Reimbursement Register (1994–2019) for menopausal hormone therapy (MHT) use [s1]. The study included women with a history of pre-eclampsia (n=31,688) and age-matched control women without pre-eclampsia (n=91,726), all of whom had given birth between 1969 and 1993 [s1]. Breast cancer risk was compared between the pre-eclamptic and control groups, and separately between hormone therapy users and non-users within each group [s1].

What it found

Breast cancer occurred less frequently among women with a history of pre-eclampsia — 6.6% (2,095 cases) — than among controls, 7.0% (6,410 cases), a modest but statistically significant difference (p = 0.02) [s1]. The age-adjusted breast cancer risk was reduced by 11% in women with a pre-eclampsia history (adjusted hazard ratio 0.89, 95% CI 0.83–0.94) [s1]. No specific clinical characteristic of the pre-eclamptic pregnancy — including its severity — predicted how large this risk reduction was [s1].

On hormone therapy, the picture was more nuanced. Hormone therapy use was associated with a comparable increase in breast cancer risk in both groups: 1.25 (95% CI 1.12–1.40) in women with prior pre-eclampsia versus 1.30 (95% CI 1.18–1.42) in controls [s1] — statistically similar increases, meaning pre-eclampsia history didn't meaningfully change how hormone therapy affected breast cancer risk. That increased risk emerged within 5 years of starting any hormone therapy in both cohorts, and was primarily associated with combined estrogen-progestogen therapy specifically — estrogen-only therapy was not associated with elevated breast cancer risk in women with a pre-eclampsia history [s1].

Why the two findings together answer a specific clinical question

Combining a protective baseline association (pre-eclampsia linked to lower breast cancer risk) with a separately quantified, comparable hormone-therapy risk increase in both groups lets the study's authors answer a question that neither finding alone would resolve: does a pre-eclampsia history change the calculus around starting hormone therapy later in life? Their answer, based on this data, is no — the hormone-therapy-associated risk increase applied equally regardless of pre-eclampsia history, meaning a woman's pre-eclampsia history doesn't function as either an added warning sign or a protective buffer against hormone therapy's separately established breast cancer association.

What might explain the pre-eclampsia protective association

The study doesn't test a specific biological mechanism directly, but pre-eclampsia is understood to involve altered placental and hormonal function during pregnancy, including different estrogen exposure patterns than typical pregnancies — a plausible, if unconfirmed, link to the modestly reduced long-term breast cancer risk observed here, consistent with the broader, well-established principle that lifetime cumulative estrogen exposure relates to breast cancer risk.

What this doesn't establish

This is an observational registry study — it can demonstrate association between pre-eclampsia history and breast cancer risk, and between hormone therapy use and breast cancer risk within each group, but it cannot establish that pre-eclampsia itself causally protects against breast cancer, since women who develop pre-eclampsia may differ from those who don't in other ways not fully captured by age-matching alone. The cohort captures women who gave birth between 1969 and 1993, meaning obstetric practices, pre-eclampsia diagnostic criteria, and hormone therapy formulations from that era may not perfectly reflect current clinical practice.

What the authors conclude

The study's authors state their finding plainly: breast cancer risk is reduced in women with a pre-eclampsia history, and because the MHT-associated risk increase is comparable regardless of that history, "pre-eclampsia is no contraindication for the use of MHT" [s1] — a direct, practically useful clinical conclusion for women and clinicians weighing hormone therapy decisions.

What to watch

Whether the biological mechanism behind pre-eclampsia's association with reduced long-term breast cancer risk is identified in future mechanistic research. This article is not medical advice; hormone therapy decisions should be made with a treating clinician who can weigh individual risk factors.

Sources

1. Reduced Breast Cancer Risk in Women With A History of Pre-Eclampsia: Implications of Postmenopausal Hormone Therapy — Journal of Clinical Endocrinology & Metabolism, 18 July 2026