Could Pregnancy Affect Your Risk of Breast Cancer? Here’s What You Need to Know

At 32 years old and six months postpartum, Amanda Boswell, a wellness coordinator from Denver, felt as though she had finally struck the elusive women-can-have-it-all balance. It was early 2014, and Boswell, an effervescent brunette with a swingy bob and a fondness for Tory Burch flats, had recently returned to work after the birth of her first child, a son named Court. Like many of her friends, she had waited to start a family until her career was firmly launched. Now she was delighted to find herself settling into the rhythms of motherhood while her career hummed steadily along.

But that delicate equilibrium was upended one evening in January 2014 when she discovered a lump the size of a walnut in her left breast. Given her situation, she wasn’t concerned: She was young and fit from going to the gym three times a week, and she had no family history of breast cancer. A week earlier, she had weaned Court and so assumed the lump was a clogged milk duct. But three months later, she was diagnosed with stage III, triple-negative breast cancer, an aggressive form of the disease that does not respond to hormone therapies. What triggered Boswell’s cancer? Her pregnancy, most likely.

Boswell is among the women whose disease comes under the category of pregnancy-associated breast cancer, or PABC, a phenomenon that includes any diagnosis during pregnancy or up to approximately five years after childbirth. (Some experts limit the definition to one year postpartum; others think it may even extend up to a decade.) Though only a small fraction of the childbearing population is affected—experts estimate that about 12,000 women in the U.S. are diagnosed with postpartum breast cancer each year—PABC affects almost half of the women under age 45 who develop breast cancer, and the mortality rate is staggering: Women diagnosed within five years of their last childbirth were almost three times more likely to die than other similarly aged breast cancer patients who had not given birth.

From gestational diabetes to postpartum depression, new and expectant mothers have long faced elevated health risks, but the idea that pregnancy itself, a condition acknowledged to provide some protection against cancer, may actually spark the disease is an especially cruel twist. However nonsensical PABC may seem, science offers an explanation in that breast cancer is, for the most part, a disease of aging. According to Pepper Schedin, Ph.D., codirector of the Young Women’s Breast Cancer Translational Program at the University of Colorado Cancer Center, “We have really compelling evidence that it takes breast cancer 10 to 20 years to develop.” In other words, older mothers are more likely to have undetected mutations, and some experts think that pregnancy may simply speed up the process of them developing into cancer. “It’s not like the pregnancy window caused your cancer—it just made it visible,” says Schedin.

As the average age of childbearing increases (first births to U.S. women age 35 and over rose ninefold between 1970 and 2012), a growing number of women are being diagnosed with the disease. While American tumor registries don’t collect detailed data on women’s reproductive cycles, reports from countries with more extensive record-keeping point to a proliferation of PABC: A 2009 study examining breast cancer in Swedish women ages 15 to 44 found that the risk of having a breast cancer detected during pregnancy and within two years of delivery rose 56 percent between 1989 and 2009, while the risk of breast cancer among women of reproductive age with no recent birth went up only 16 percent. Without such records stateside, experts at the leading cancer facilities in the U.S. can cite a rise only from anecdotal evidence. “We are seeing pregnancy and cancer intersecting more often, especially as IVF and other reproductive technologies are making pregnancy in women’s late 30s and early 40s more common,” says Jennifer Keating Litton, MD, an oncologist at MD Anderson Cancer Center in Houston. (There is no known causality between IVF and breast cancer.)

The relationship between pregnancy and cancer is a complicated one. On the one hand, pregnancy after 30 slightly raises a woman’s risk of developing the disease for about a decade after giving birth. Yet having a baby before the age of 30 is linked with a lowered incidence of breast cancer post-menopause, when it is most likely to occur. On the whole, says Virginia Borges, MD, MMSC, Schedin’s codirector at the University of Colorado Cancer Center, evidence is weighted in favor of pregnancy. “Hands down, pregnancy is ultimately protective against breast cancer for the majority of women at risk, but there is this subset for which pregnancy is not protective and is probably on some level causative.”

Researchers are still trying to understand the physiological factors specific to pregnancy that cause PABC, but one thought is that a fetus might compromise the immune system, creating an environment that allows a baby, an infection, or a cancer to grow. Then there are increased levels of estrogen and progesterone during pregnancy; both hormones have been proven to promote cancer growth. Another risky stage is involution, when the breast returns to its pre-lactation state after weaning (or directly after pregnancy if nursing doesn’t occur). The process’s risks remain five years or more postpartum, according to Borges and Schedin. Their studies have shown that involution has properties of inflammation that may explain why metastasis is more likely to occur.

Another danger common to PABC is delayed diagnosis, since breast lumps in postpartum women are often thought, by both women and their doctors, to be mastitis (an infection of the breast that results in pain or swelling) or benign changes in breast tissue. In June 2014, Alexa Goins Hu, a 37-year-old stay-at-home mom in Framingham, Massachusetts, found that her right breast had become engorged after six months of nursing her youngest daughter, Vivienne. “It didn’t feel like there was a strange lump,” she says. “It just seemed like my breast was extra full.” When she contacted her ob-gyn’s office, the doctor on call said it was likely a clogged duct and suggested warm compresses and other remedies. But the discomfort returned two months later, so Hu had a mammogram and ultrasound. In a matter of days, she was diagnosed with breast cancer. “I was shocked,” she recalls. “I was 36.”

In assessing the effects associated with delaying childbirth, doctors are wary of fearmongering. Relatively speaking, the chance of a PABC diagnosis is actually quite rare: Less than 1 percent of new moms each year will be diagnosed with the disease. And yet, the facts still merit thoughtful consideration—especially because, according to experts, the best defense against PABC may have everything to do with a heightened awareness of the issue. “People should continue having pregnancies, but be vigilant about symptoms and report to your doctor immediately any lumps, bumps, or concerning symptoms in your breasts or under arms,” says Ann Partridge, MD, founder and director of the Program for Young Women With Breast Cancer at the Dana-Farber Cancer Institute in Boston. Monthly self-breast exams remain a powerful tool, as does a proactive approach. Partridge counsels women to exercise regularly and maintain a healthy body weight throughout their lives. She also advises drinking in moderation (no more than one glass of wine, or the equivalent, per day) and avoiding exposure to exogenous estrogen, which is found in hormone replacement therapy and can also be found in some pesticide-treated produce and hormone-fed meat. As Schedin stresses, a balanced diet and overall lifestyle “can significantly reduce your chance of getting breast cancer and can help your body fight back when challenged.”

For those like Boswell, who had her left breast removed in September 2014, followed by several months of radiation therapy, PABC is a life-changing reality—but one that, caught early, can be faced with hope. Today she is cancer-free and her risk of recurrence is slim. She feels optimistic about the future and plans to have more children. Now two and a half, Court, all blue eyes and blond hair, is walking and talking. And though PABC presented her life with an unexpected twist, Boswell has managed to find the silver lining in the experience. “Cancer was terrible, but it made me appreciate being a mom so much more,” she says. “I would do it all again for my son.”

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