A new study from researchers at UCLA has found that women who are diagnosed with breast cancer within three years after giving birth may have a more aggressive form of the disease than women who have never had children.
The risk appears to be highest during the first year after childbirth.
The findings, published in npj Breast Cancer, suggest that the months and years after pregnancy may be an important period when some breast cancers behave differently.
The research could help doctors better understand which patients may benefit from closer monitoring or more personalized treatment.
Breast cancer is becoming more common among younger women, and scientists have been trying to understand why.
One possible factor is that many women are having their first child later in life. Pregnancy causes major changes in breast tissue, and researchers have suspected that these changes may affect how breast cancer develops after childbirth.
Until now, however, experts have not agreed on how long the higher-risk postpartum period lasts. Some studies have defined it as one or two years after delivery, while others have suggested it may continue for as long as five or even 10 years.
To investigate this question, the UCLA team studied 385 women aged 45 or younger who had early-stage hormone receptor-positive, HER2-negative breast cancer. All of the women were treated at UCLA between 2011 and 2024.
Each woman’s tumor was tested using the Oncotype DX Breast Recurrence Score. This widely used genetic test examines the activity of 21 genes linked to the chance that breast cancer will return. It also helps doctors decide whether chemotherapy is likely to be beneficial.
The researchers compared women who had never given birth with women who developed breast cancer at different times after childbirth.
They found that women diagnosed during the first year after giving birth had significantly higher recurrence scores. These higher scores suggest that their cancers had biological features linked to a greater risk of returning after treatment.
Women diagnosed during the second and third years after childbirth also had higher scores, although the increase was smaller.
Overall, women diagnosed within three years after giving birth were nearly three times more likely to have a higher recurrence score than women who had never been pregnant. Their tumors were also more likely to be high grade, meaning the cancer cells looked more abnormal under a microscope and were more likely to grow and spread quickly.
Interestingly, women diagnosed more than three years after childbirth did not show the same pattern.
The study also found that common measures used to assess breast cancer, such as tumor size or whether cancer had spread to nearby lymph nodes, did not always reflect these biological differences. Genetic testing appeared to provide additional information that routine examinations might miss.
Despite the more aggressive tumor features, the researchers found encouraging news. After an average follow-up of about four years, women diagnosed within three years of childbirth did not have worse survival or higher recurrence rates than other women in the study.
The researchers believe this may be because many of these women received more intensive treatment, including chemotherapy, hormone-blocking therapy, and newer targeted medicines. These treatments may have helped reduce the impact of the more aggressive tumors.
Although larger studies with longer follow-up are still needed, the findings suggest that a woman’s recent pregnancy history may be an important factor when evaluating breast cancer.
Better understanding how breast cancer behaves after childbirth could help doctors identify higher-risk patients earlier and provide the most appropriate treatment for each individual.
