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Breast cancer diagnosed within three years of childbirth, especially the first year, may be biologically more aggressive, a study suggests.
The findings add to evidence that postpartum breast cancer may be a distinct form of the disease.
They also suggest the period of greatest biological risk may occur earlier than previously thought.
The study was led by investigators at the UCLA Health Jonsson Comprehensive Cancer Center.
Dr Nimmi Kapoor, associate professor of surgery at the David Geffen School of Medicine at UCLA and senior author of the study, said: “We’ve long recognised that breast cancers diagnosed after pregnancy can behave differently, but we haven’t known when that increased risk is biologically strongest.
“Our findings suggest that the first one to three years after childbirth represent an important window when some tumours may have more aggressive characteristics.”
Breast cancer rates among younger women have been rising, and scientists have been investigating whether women having their first child later may help explain some of the trend.
Pregnancy causes major changes in breast tissue. Previous studies have found that cancers diagnosed soon after childbirth are more likely to have aggressive features and worse outcomes.
Researchers have not agreed on how long the period of increased risk lasts. Some studies define postpartum breast cancer as occurring within one or two years of delivery, while others extend the period to five or even 10 years.
To better define the period of risk, the team studied whether tumour biology varied according to the time since a woman’s most recent childbirth.
The study involved 385 women aged 45 or younger with early-stage, hormone receptor-positive and HER2-negative breast cancer who were treated at UCLA between 2011 and 2024.
Hormone receptor-positive cancers grow in response to hormones such as oestrogen or progesterone. HER2-negative cancers do not have unusually high levels of a protein that can promote tumour growth.
Each tumour had been assessed using the Oncotype DX Breast Recurrence Score, a genomic test that measures the activity of 21 genes linked to the risk of cancer returning and the potential benefit of chemotherapy.
Researchers grouped the women according to the time between their last childbirth and breast cancer diagnosis.
They compared women who had never given birth with those diagnosed at different intervals after childbirth.
The team then examined whether recurrence scores and other tumour features differed between the groups and whether any patterns remained after accounting for factors including age and lymph node status.
Women diagnosed within the first year after childbirth had significantly higher recurrence scores than those who had never given birth.
This suggested biological features associated with a higher risk of the cancer returning.
Scores were also higher, but to a lesser extent, among women diagnosed during the second and third years after delivery.
Women diagnosed within three years of childbirth were nearly three times more likely to fall into a higher recurrence score category than women who had never given birth.
They were also more likely to have higher-grade tumours, meaning their cancer cells appeared more abnormal and potentially more aggressive under a microscope.
Women diagnosed more than three years after childbirth did not show the same consistent increase in recurrence scores.
The findings also suggest that standard clinical measures, including tumour size and whether the cancer has reached the lymph nodes, may not fully capture the differences in this group.
Gene expression testing appeared to identify biological risk that was not always reflected in routine examination of tumour tissue.
The researchers said reproductive history could therefore provide additional context when genomic test results are interpreted in younger patients.
Despite the more aggressive genetic features, women diagnosed within three years of childbirth did not have significantly worse short-term outcomes.
After about four years of follow-up, recurrence and survival rates were similar to those among other patients in the study.
Researchers said one possible explanation was that women with higher-risk tumours received more intensive treatment, including chemotherapy, ovarian function suppression and newer targeted therapies.
The findings also suggest that aggressive tumour biology does not necessarily lead to worse short-term outcomes when patients receive effective treatment.
The researchers said larger studies involving several institutions and longer follow-up periods are needed to confirm the findings.
They added that postpartum status may need greater consideration when breast cancer is assessed and treated in younger women.
Kapoor said: “Our findings suggest that the years immediately following childbirth represent a unique biological window for some breast cancers.
“Understanding why these tumours behave differently may help us better identify patients who need closer monitoring or more tailored treatment approaches.”
