A growing number of health advocates are raising concerns about possible links between the estrogen-like chemical BPA and breast cancer.
Consumer concern about BPA, or bisphenol A, has led manufacturers to remove it from baby bottles and infant formula packaging.
Still, BPA also could pose a risk to children long before they take their first sip of milk, according to a September report from the Breast Cancer Fund, an advocacy group. Babies also are exposed in the womb, the report finds.
A developing fetus is especially vulnerable during the first 11 weeks of pregnancy, says co-author Sharima Rasanayagam, director of science at the Breast Cancer Fund. "Everything is being developed" at this stage, she says. "The building blocks are being laid down for future health."
The report cites 60 animal and human studies, which link prenatal BPA exposure to an increased risk of a variety of health problems, from breast cancer and prostate cancer to decreased fertility, early puberty, neurological problems and immune system changes.
In a September paper, too new to be included in the report, Tufts University's Ana Soto found that BPA increased the risk of mammary cancers in rats. In two studies of rhesus monkeys published last year, other researchers found that BPA disrupted egg development, damaged chromosomes and caused changes in the mammary gland that made animals more susceptible to cancer.
Soto says it's possible that prenatal BPA exposure makes fetuses more sensitive to estrogen, a hormone that drives the growth of most breast cancers. In that way, BPA could indirectly increase the risk of breast cancer later in life. She notes that even small changes in prenatal estrogen exposure - such as that produced by the extra placenta in a uterus containing fraternal twins - increases the risk of breast cancer in girls and prostate cancer in boys.
In a separate action, the American College of Obstetricians and Gynecologists and the American Society of Reproductive Medicine also have released a joint report on the broader issue of prenatal exposure to toxins, from BPA to pesticides and other chemicals. That September report notes that in utero exposure to environmental chemicals has been linked to miscarriage and stillbirth, impaired fetal growth and low birth weight, preterm birth, childhood cancers, birth defects, intellectual impairment and thyroid problems.
In 2011, the American Medical Association labeled BPA an "endocrine-disrupting agent" because of evidence suggesting that it disrupts the body's normal hormonal regulation.
In 2009, the Endocrine Society - a group of doctors and researchers specializing in the hormonal system - called hormone-disrupting chemicals such as BPA a "significant concern for public health," possibly causing infertility, cancer and malformations.
"Every pregnant woman in America is exposed to many different chemicals in the environment," says Jeanne Conry, president of the obstetrics-gynecology group.
More than 90% of American have BPA in their bodies, research shows. Ten studies have found BPA in fetal tissue, including umbilical cord blood, as well as in amniotic fluid, the Breast Cancer Fund report notes.
BPA was developed in the 1930s as an estrogen-replacement therapy. Researchers stopped developing BPA as an estrogen, however, because another synthetic hormone, DES, or diethylstilbestrol, was far more potent.
Conry says she's concerned that exposure to BPA could, like DES, change the way that a developing fetus reacts to estrogen for the rest of its life.
Millions of pregnant women took DES from 1941 to 1971 to prevent miscarriage, until studies found that women exposed to DES before birth had a high rate of rare vaginal cancers. Studies later linked DES to breast cancer, as well.
While concern over BPA has led many manufacturers to stop using the chemical in plastic bottles, it remains widely used in other plastics, such as bicycle helmets, eyeglasses and medical equipment and the linings of metal food cans. BPA also is found in the coatings on many cash-register paper receipts.
Since 2011, the Breast Cancer Fund has campaigned to persuade food companies to stop using BPA.
Campbell's Soup last year announced that it will phase out BPA, but has not yet announced when that will happen, or what material it will use instead. Eden Foods has sold its beans in BPA-free cans since 1999. It now sells tomatoes in glass jars, which have lower levels of BPA than traditional cans.
The American Chemistry Council, an industry group, notes that BPA plays an important role in food safety, because it "helps to extend a product's shelf life and protects food from contamination and spoilage."
"BPA is one of the most tested substances in use today and regulatory agencies around the world have repeatedly found that the evidence does not show a connection between typical exposure levels and health effects or disease," says Jayne Morgan, chief medical officer at the American Chemistry Council. "Women rely on their physicians for sound medical advice and access to reliable information. Creating confusion and alarm among expectant mothers will distract from the well-established steps doctors recommend to support a healthy pregnancy and a healthy baby."
The Food and Drug Administration's official statement on BPA says that it is "safe at the very low levels that occur in some foods.This assessment is based on review by FDA scientists of hundreds of studies including the latest findings from new studies initiated by the agency."
However, the FDA also has expressed "some concern" about the potential effects of BPA on the brain, behavior, and prostate gland in children, both before and after birth. The FDA banned BPA in baby bottles in 2012, after most manufacturers already had stopped using it. Earlier this year, the FDA formally banned BPA in infant formula packaging, also after formula manufacturers already had abandoned the chemical.
Manufacturers of metal cans say there's no clear evidence that BPA linings cause harm.
John Rost, chairman of the North American Metal Packaging Alliance, says that recent "studies, coupled with the extensive body of research on BPA that has been thoroughly reviewed by FDA, and multiple international regulatory bodies, continue to reaffirm that the trace amounts of BPA found in metal food and beverage packaging does not represent a health risk to humans at any age or stage of development."
A growing number of studies have linked prenatal BPA exposure to genital changes in babies, as well as behavioral changes in children, Rasanayagam says.
Still, some breast cancer researchers say there's not yet enough research to know if BPA really increases breast cancer risk, and if removing it from consumer products will reduce that risk.
"Yes, we should do the research," says surgeon Susan Love, author of Dr. Susan Love's Breast Book. "But I don't think we should be jumping ahead of the data."
Yet Rasanayagam notes that doctors will never have the sort of scientific proof about environmental chemicals that they have for other products, such as new drugs. First, that's because it's unethical to deliberately expose women and their fetuses to a potentially harmful substance. Second, breast cancer can take 60 or 70 years to develop. So even a very large trial would take decades to produce results.
Soto says that the evidence for regulating the chemical is as good as it's going to get. "It is no longer a problem of science," says Soto, of Tufts University School of Medicine. "It's a problem of policy."
In the face of uncertainty, Conry says doctors and their patients can take sensible steps, such as avoiding plastics made with BPA and not heating plastic in the microwave, which can cause chemicals to leach into food.
Yet BPA is too ubiquitous for women to completely avoid, Conry says. That's why the USA should "shift the burden of proof" from individuals to manufacturers and regulators, who should make sure that pregnant women and others aren't exposed to hazardous chemicals. She notes that there are about 84,000 chemicals in use today, and 700 others are introduced each year. Women and their doctors can't be expected to be experts on all of them, she says.
"We shouldn't be releasing these chemicals into the environment until they have had adequate study," Conry says. "The burden shouldn't be on the consumer each time these issues come up. That's more than the individual can take on."