Becoming pregnant after a diagnosis of breast cancer does not raise the risk of the cancer returning, said the largest study of its kind to date, released Saturday at a major cancer conference.

The study included 1,207 women under age 50 who had breast cancer that had not spread elsewhere in the body.

Most of the women in the study (57 percent) had estrogen receptor (ER) positive cancer, a type in which the tumors are fueled by the hormone estrogen.

Some doctors have raised concerns that these women might face a higher risk of cancer recurrence if they were to become pregnant, due to hormonal changes during gestation.

A total of 333 of the women became pregnant during the study, within in an average of about 2.4 years following diagnosis and treatment.

After a follow up of 10 years, researchers found “no difference in disease-free survival between women who became pregnant and those who did not, irrespective of ER status,” said the report released at the American Society of Clinical Oncology meeting.

Pregnancy also showed surprising survival benefits for women who had survived ER-negative breast cancer, it said. 

These women had a 42 percent lower chance of dying than those who did not become pregnant. 

“Our findings confirm that pregnancy after breast cancer should not be discouraged, even for women with ER-positive cancer,” said lead study author Matteo Lambertini, a medical oncologist at the Institut Jules Bordet in Brussels. 

“It’s possible that pregnancy could be a protective factor for patients with ER-negative breast cancer, through either immune system mechanisms or hormonal mechanisms, but we need more research into this.”

Breast cancer is the most common cancer in women of reproductive age.

About half of young women recently diagnosed with breast cancer say they are interested in bearing children, but research shows less than 10 percent become pregnant after treatment.