The pause that was feared most turned out to cost the least.
For a young woman diagnosed with hormone receptor–positive breast cancer, the treatment math has long been brutal. Endocrine therapy — the medication that suppresses estrogen and keeps the cancer at bay — runs for five years at minimum, and increasingly for ten in higher-risk cases. For a woman in her mid-thirties who hasn't yet had children, that timeline doesn't just reshape her treatment plan. It reshapes her life.
The POSITIVE trial was built around that reality. Led by the International Breast Cancer Study Group and spearheaded by Ann Partridge, MD, of Dana-Farber Cancer Institute, the study enrolled 516 premenopausal women with hormone receptor–positive, HER2-negative early breast cancer who wanted to try for a pregnancy. The protocol allowed them to complete 18 to 30 months of endocrine therapy, then pause — for up to two years — to conceive, carry a pregnancy, deliver, and breastfeed if they chose, before resuming treatment. The question the trial was designed to answer: would that pause cost them their lives?
The first results, published in 2023, were reassuring. Now, with participants followed for a median of 71 months, the updated data presented at the 43rd Annual Miami Breast Cancer Conference have deepened that reassurance considerably. The short version: most women had a baby, and the cancer outcomes were not worse.
Of the 497 evaluable participants, 377 — fully 76 percent — became pregnant at least once, producing a total of 589 pregnancies and 440 live births. Eighteen sets of twins were among them, and 75 women had more than one live birth. Obstetric complication rates landed squarely within population norms for women of comparable age: roughly 2.5 percent experienced pregnancy complications, 8.6 percent had low-birth-weight infants, and 1.8 percent had infants with birth defects — figures consistent with baseline rates in Western countries for an older maternal cohort.
The oncologic picture was equally steady. When POSITIVE participants were compared against a matched cohort drawn from the SOFT and TEXT trials — controlled for tumor size, nodal status, and prior treatment — the outcomes tracked closely. Breast cancer–free interval events at 71 months were 12.3 percent in the POSITIVE group versus 13.2 percent in the comparison group, a hazard ratio of 0.88. Distant recurrence–free interval events were 6.2 percent versus 8.3 percent, a hazard ratio of 0.67. Neither figure suggested that pausing therapy for pregnancy made things worse. If anything, the numbers tilted the other way.
A landmark analysis embedded within the trial data pressed the question further. Among participants who had no evidence of disease at enrollment, those who became pregnant within 18 months were compared directly to those who did not — a comparison that isolated the effect of pregnancy itself, since both groups had wanted to conceive. The breast cancer–free interval was not statistically different between the two groups, and the hazard ratio favored the women who had gotten pregnant, at 0.64. Partridge described this as effectively closing the argument that pregnancy triggers recurrence.
Assisted reproductive technology was used by 43 percent of evaluable participants, and embryo transfer more than doubled the odds of achieving pregnancy. Oncologic outcomes among ART users were comparable to those who conceived without assistance. Seventeen percent of participants underwent ovarian stimulation during the trial itself, with no signal of harm — though Partridge was careful to note the trial was not powered to evaluate that endpoint definitively. Data on fertility preservation undertaken before trial enrollment were similarly encouraging: women who had undergone ovarian stimulation prior to joining the study did no worse than those who had not.
Adherence to resuming endocrine therapy after pregnancy was notably strong. By the 71-month mark, 72 percent of participants had restarted treatment — a figure that compares favorably to general population data showing roughly half of young patients discontinue endocrine therapy within five years. The trial's structured, protocol-driven approach appears to have supported follow-through in a way that routine clinical care often does not.
The results are not a blanket green light. Among node-positive patients with four to nine involved nodes, the 71-month recurrence rate reached 23 percent — a reminder that underlying disease risk doesn't dissolve because a woman chooses to pursue pregnancy. High-risk cases require careful, individualized counseling. The trial also reviewed data from a separate matched analysis of 4,732 BRCA mutation carriers under 40, drawn from a worldwide collaboration: one in five conceived within ten years of diagnosis, and pregnancy was not associated with decreased disease-free survival.
What the POSITIVE trial has done, at its core, is replace fear with evidence. For years, the guidance young women received was cautious by default, because the data simply weren't there. Now they are — prospective, trial-level, followed for nearly six years. The conversation between oncologist and patient about fertility no longer has to begin with an apology.
Notable Quotes
Women have told me they feel their life is like a ball and chain when they learn they have to wait five or even ten years to attempt pregnancy.— Ann Partridge, MD, Dana-Farber Cancer Institute, principal investigator of the POSITIVE trial
If anything, the women who had a pregnancy clearly had fewer recurrences, at least numerically — that's kind of the nail in the coffin that pregnancy itself would cause a recurrence.— Ann Partridge, MD, presenting at the 43rd Annual Miami Breast Cancer Conference
The Hearth Conversation Another angle on the story
What was actually being tested here — the safety of pregnancy, or the safety of stopping the medication?
Both, really, but the medication pause was the mechanism. You can't get pregnant safely on endocrine therapy, so the question was whether stopping it long enough to conceive and deliver would give the cancer a window to come back.
And did it?
No. The recurrence rates in women who paused were comparable to matched controls who never paused. In fact, the numbers slightly favored the women who took the break, though the differences weren't large enough to be statistically definitive.
That landmark analysis — the one comparing women who got pregnant to those who wanted to but didn't — why does that design matter?
Because it removes the selection effect. If you just compare pregnant women to all other patients, you're mixing in women who never wanted children, or who were too sick to try. This way, both groups wanted the same thing. The only difference was whether they achieved it. That makes the comparison much cleaner.
Seventy-six percent had a live birth. Is that high, given the circumstances?
It's striking. These are women who had cancer, many of whom had chemotherapy, with a mean age of 37. The IVF community had long assumed pregnancy after breast cancer was largely not feasible. This data challenges that directly.
What about the women who used assisted reproduction — did that carry any extra cancer risk?
Not according to this data. Oncologic outcomes for ART users were comparable to those who conceived naturally. That's important because ART involves hormonal stimulation, which had been a theoretical concern in hormone-sensitive cancers.
The adherence numbers stood out to me — 72 percent back on treatment at five-plus years. Why is that notable?
Because in the general population, roughly half of young patients have stopped endocrine therapy by year five. The trial's structure — the expectation of resumption, the follow-up — seems to have kept people on track in a way that routine care often doesn't.
What's the limit of what this trial can tell us?
It wasn't designed to evaluate every subgroup. The ovarian stimulation data, for instance, was uncontrolled. And for women with high-risk node-positive disease, the recurrence rates were still substantial — 23 percent at 71 months. Pregnancy doesn't change the underlying biology of an aggressive tumor.
So what changes in the clinic because of this?
The conversation. Oncologists now have prospective trial data to offer instead of cautious guesswork. That's not a small thing when you're sitting across from a 35-year-old who just got a diagnosis and is asking whether she'll ever have a family.