Women who have had early-stage breast cancer and become pregnant do not have a greater chance of recurrence and death than those who do not get pregnant, according to results released Saturday from the largest study to ever explore the issue.
The study is the first to focus specifically on the safety of pregnancy for women whose cancers are fueled by estrogen.
Researchers said their conclusions should allay concerns among some doctors and patients that pregnancy, which results in a surge in estrogen levels, could put these women at risk by encouraging the growth of any cancer cells that might remain in the body after treatment.
"Over the long term, having a pregnancy is safe," said lead study author Matteo Lambertini, an oncologist at the Institut Jules Bordet in Brussels. "We were very happy to see that."
The retrospective study involved 1,200 women in Massachusetts and across Europe who were followed for a decade.
Almost 60 percent of the women had estrogen-receptor-positive cancer.
The results were presented as an abstract at the annual meeting of the American Society of Clinical Oncology in Chicago.
In a 2013 article in the Journal of Clinical Oncology, the same researchers reported their five-year findings: Pregnancy did not increase the risk of a cancer recurrence.
But they decided it was important to follow the patients over a longer time and, as they reported Saturday, the 10-year results showed no difference in recurrence.
With many young women having children at later ages, it's no longer so unusual for them to develop breast cancer before they have completed their family plans, the researchers said.
About half of younger women with the disease say they want to have children, though only 10 percent actually become pregnant after treatment, partly reflecting fertility problems caused by the therapies.
The women in the study were diagnosed under the age of 50 and before 2008; in no cases had the cancer spread beyond the breast.
The researchers matched each patient who became pregnant with ones who did not but had similar tumor and treatment characteristics.
The median time from diagnosis to conception was 2.4 years.
Jennifer Litton, an oncologist at MD Anderson Cancer Center in Houston, who wasn't involved in the study, called the latest data "very reassuring" but added that young women who have had breast cancer still face several difficult issues.
Those with estrogen-receptor-positive cancer, for example, often take tamoxifen after surgery to reduce the risk of recurrence.
But the therapy is recommended for five to 10 years, and women can't get pregnant while taking it because the medication can cause birth defects.
What isn't clear is whether women who temporarily interrupt their tamoxifen treatment to have a baby face a higher risk of recurrence.
That question is being tackled by a big clinical trial called POSITIVE, which is being conducted at dozens of locations in the United States, Australia, Canada, Europe and Japan. No data from that study will be available for some years, Litton said.
Another tough issue for younger breast-cancer survivors is the effect of cancer treatment on their fertility.
Because chemotherapy and tamoxifen can impair women's ability to have children, doctors advise patients to have fertility counseling before starting either treatment.