The new effort, known as the NorthMom Study, was described in a study protocol published in BMC Public Health. It is an observational, population-based cohort nested within NorthPop, designed to follow women from their pregnancies into the age at which routine breast cancer screening begins. Rather than waiting decades to count cancer diagnoses, the investigators are focusing on intermediate markers—most importantly mammographic breast density—that are strongly associated with future breast cancer risk and can be measured at scale through Sweden’s national screening program.

The scientific rationale rests on a concept that cancer biologists call windows of susceptibility. Breast tissue does not respond uniformly to environmental insults throughout a woman’s life; instead, there are discrete periods—prenatal development, puberty, pregnancy, and menopause—when the mammary gland is undergoing rapid structural and hormonal change and is correspondingly more vulnerable. During pregnancy in particular, breast tissue undergoes dramatic maturation driven by surges of hormones, making it sensitive to compounds that can interact with hormone receptors, including endocrine-disrupting chemicals. An exposure that perturbs these processes, the hypothesis goes, could trigger biological effects such as alterations in breast density, shortened breastfeeding duration, or persistent changes in gene expression and metabolism that mediate the path from exposure to eventual disease.

Breast density itself—the proportion of fibroglandular tissue within the breast—is one of the strongest known independent risk factors for breast cancer, and it also complicates screening by masking underlying tumors on mammograms. Density is shaped by hormonal, genetic, and environmental influences, yet surprisingly few studies have examined how environmental exposures, particularly those occurring during windows of susceptibility, relate to density measured later in life. NorthMom is explicitly designed to fill this gap by linking prospectively collected pregnancy exposures to density measurements taken when participants turn 40 and attend their first screening mammograms.

The exposure data available to the researchers are unusually rich. During pregnancy, NorthPop collected detailed questionnaire information on diet, physical activity, medication use, mental health, and home and work environments. For all participants, the cohort holds geospatial estimates of ambient air pollution, traffic noise, and residential access to green spaces. For subsets of women, levels of persistent organic pollutants were measured in plasma samples banked during pregnancy, and the team plans eventually to perform comprehensive chemical exposomics analyses on plasma from all enrolled women. Because these data were collected before any outcomes existed, the study avoids the recall bias that plagues much of environmental epidemiology.

Recruitment into NorthMom follows a straightforward protocol. Every woman enrolled in NorthPop is invited to participate in the year after her 40th birthday, provided she still lives in Västerbotten, has not withdrawn from the parent cohort, and is not currently pregnant. Those who agree visit the Clinical Research Center at the University Hospital of Umeå, where a nurse draws a venous blood sample that is separated into plasma, buffy coat, and erythrocyte fractions and stored at minus 80 degrees Celsius in the regional biobank. Participants also undergo dual-energy X-ray absorptiometry, or DXA, a scan that quantifies body fat percentage and distribution far more precisely than body mass index, and they complete a short questionnaire on reproductive history and hormone therapy or contraceptive use.

Breast density is assessed two ways. Screening mammograms obtained through Sweden’s national program are classified by a consultant breast radiologist using both the fourth and fifth editions of the Breast Imaging Reporting and Data System, and independently analyzed with LIBRA, an open-source software tool that produces automated measures of percent density and dense area in square centimeters. Using both subjective and automated methods allows the team to cross-validate the software and to express density as a continuous variable, which boosts statistical power. For women with multiple screening occasions, the study will also track how density changes over time using mixed-effects models, testing whether pregnancy exposures are associated not just with density at a single point but with the rate at which density evolves through the menopausal transition.

The statistical planning is already in place. Based on pilot data showing a standard deviation of roughly 19.7 percentage points for breast density, the investigators calculate that 1,049 women would give the study 80 percent power to detect a 1.7-percentage-point difference in density per one-standard-deviation increase in exposure, at a two-sided significance level of 0.05. Analyses will employ linear regression for continuous density, ordinal and logistic regression for BI-RADS categories, and repeated-measures models for longitudinal trajectories. Early recruitment figures show 36.6 percent of invited women enrolling—442 of 1,207 in the first year—and ongoing funding is expected to carry recruitment at least through 2028, by which time more than 3,000 NorthPop mothers will have turned 40 and around 1,000 participants should be enrolled.

The study’s design offers several distinctive strengths. DXA scans capture adiposity with a granularity that BMI cannot match, addressing a persistent weakness in breast cancer epidemiology where crude proxies often obscure the true role of body fat. The link to NorthPop means exposure data and biobanked pregnancy samples already exist, dramatically reducing costs and enabling biomarker analyses of the internal exposome—proteomic, metabolomic, and epigenetic signatures that may reveal how environmental chemicals leave durable biological footprints. Sweden’s comprehensive national registers, including those covering cancer diagnoses, prescribed drugs, causes of death, and family relationships, allow deep longitudinal follow-up, and linkage with the long-running Northern Sweden Health and Disease Study could eventually provide exposure and blood sample data spanning pregnancy, perimenopause, menopause, and beyond for a substantial share of participants.

The investigators are candid about limitations. The cohort’s ultimate size, likely around a thousand women, is too small to use breast cancer diagnoses themselves as a primary outcome. But because studies of environmental effects on breast cancer intermediates such as density are scarce, the researchers argue that this unusual setting is precisely what makes new discoveries possible, with density serving as a defensible surrogate for cancer-specific effects. If the hypothesis holds—that chemical and physical exposures during pregnancy leave measurable imprints on breast tissue decades later—NorthMom will have helped expose one of the most overlooked chapters in the environmental history of breast cancer, and it will have done so by following the mothers of a birth cohort every bit as carefully as their children.

Subject of Research: A population-based cohort study examining how environmental exposures during pregnancy affect breast cancer risk factors, particularly mammographic breast density, in women over 40

Article Title: The NorthMom study: population-based follow-up of women in a birth cohort to assess pregnancy exposures and effects on breast cancer risk factors

Article References: Fredriksson, A., Lundberg-Ulfsdotter, R., Vinnars, M.-T., Oudin, A., Van Guelpen, B., West, C. E., Domellöf, M., Wu, W. Y.-Y., Dembrower, K., & Harlid, S. (2026). The NorthMom study: population-based follow-up of women in a birth cohort to assess pregnancy exposures and effects on breast cancer risk factors. BMC Public Health, 26(1), Article 2670. https://doi.org/10.1186/s12889-026-29622-0

Image Credits: AI Generated

DOI: 10.1186/s12889-026-29622-0

Keywords: breast cancer, breast density, pregnancy, environmental exposures, endocrine-disrupting chemicals, air pollution, birth cohort, mammography, epidemiology, exposome, women's health, PFAS