A breast cancer diagnosis comes with a variety of difficult treatments, from chemo and radiation to surgery. For patients with hormone-sensitive breast cancers, though, there’s still more treatment to be had after all that. Patients with these forms of the disease usually undergo hormone therapy for at least five years to lower the odds of recurrence. For those wanting to have children, though, it may be necessary to pause the treatment and then resume it after their child is born. A new study finds this resumption usually doesn’t happen.

Researchers at Stanford Medicine recently looked into how often women who pause hormone therapy to become mothers end up taking up the therapy again, after a clinical trial found that most do. The POSITIVE trial of several hundred survivors found that 73% of women who paused their treatment for pregnancy ultimately resumed it, and only 9% had experienced a recurrence within three years of childbirth. The Stanford study, meanwhile, wanted to see if that held true outside of a clinical trial. 

Dr. Allison Kurian, the study’s senior author and professor of medicine and of epidemiology and population health, explains, “It’s always important to understand how guidelines play out in a real-world setting. Clinical trial participants tend to be highly motivated and may be more likely to adhere to treatment recommendations. But we are interested in outcomes for all our patients — that’s where the rubber really meets the road.”

Their study, published in JAMA Oncology, had much different outcomes than the clinical trial. They looked at health data from 215 women who had been diagnosed with breast cancer between 2000 and 2024 who had gone on to become pregnant. Their median age at diagnosis was 33. Of this group, 161 were eligible for hormone therapy, and 113 of them decided to undergo the treatment. However, only 36 completed at least five years.

Of the 81 patients who paused the treatment for pregnancy, for an average of 21 months, only 36 ended up resuming it. Further, nearly 20% experienced a recurrence within the next 10 years.

Dr. Julia Ransohoff, the study’s first author and clinical fellow in hematology and oncology, says, “This is about twice as high as we would expect in these younger women, and it suggests that failing to meet these therapeutic guidelines is contributing to poorer outcomes. We need to do a better job of understanding barriers to resumption of care and help this population follow through with their treatment, particularly when breast cancer rates in this age group are increasing.”

The team doesn’t know the reasons why patients chose not to resume their treatment, but they believe it may have to do with the side effects, including menopausal symptoms and impacts to sexual health. They hope this research can lead to bigger studies that look into treatment barriers and how they may differ based on a patient’s location or background.