Even as women choose to have babies later in life, more are having trouble conceiving, and the chemical BPA might be partly to blame, suggests a new study.
Mice that were exposed to tiny amounts of the common chemical in the womb and shortly after birth had no problems getting pregnant early in their reproductive lives, the study found. But the animals were less likely to get pregnant as they aged compared to animals that had not been exposed to BPA, and they gave birth to smaller litters as time wore on.
People come in contact with BPA, also known as bisphenol A, through cash register receipts, canned foods and beverages, hard plastic bottles, kitchenware, DVDs and many other sources. Just about all of us have BPA in our bodies, where it can interfere with the action of estrogen and other hormones.
That process, accumulating evidence suggests, might lead to all sorts of negative health consequences, including some cancers, behavioral issues, and developmental problems.
Previous studies have looked for links between BPA and fertility, but those studies have only asked whether young animals might have trouble getting pregnant on the first try after being exposed to the chemical, said Ana Soto, an endocrinologist and developmental biologist at Tufts University in Boston.
"We asked the question of whether or not the animals would reproduce well during a period that corresponds to a large portion of adulthood," Soto said, "and whether or not problems due to premature reproductive aging would manifest in the second or third pregnancies."
Soto and colleagues began by allowing a group of untainted mice to mate freely. As soon as females were pregnant, they were assigned to one of four groups that received daily doses of a BPA-containing solution, in amounts ranging from none of the chemical to levels equivalent to those normally found in people.
When offspring were born, they nursed for three weeks. Then, five weeks later, about 20 females from each group were paired with males that had not been exposed to BPA. An uninhibited mating frenzy followed.
It took the same amount of time -- a little more than three weeks -- for females in all four exposure-groups to give birth for the first time, the researchers report today in the journal Environmental Health Perspectives.
By week 16, though, females that had been exposed to both the lowest and highest levels of BPA during development had delivered an average of 50 pups each -- 25 percent fewer than the 67 pups delivered by BPA-free mothers.
Overall, mothers that had received the highest doses of BPA early in life had fewer total pregnancies, with just 35 percent of them delivering six or more litters over the eight-month study, compared to 76 percent of the BPA-free mothers.
"To me, the most important aspect of this is the really serious implications for humans," said Patricia Hunt, a reproductive biologist at Washington State University, Pullman. She added that the new study confirms what she and others have been predicting a study like this might find.
"Everyone knows our fertility starts to decline around age 35," Hunt said. "If there's an effect like this where something accelerates our reproductive decline so that instead of starting at 35, it started at 30, that would have serious repercussions for us. It would mean that more and more couples would face infertility and not be able to have the families they hoped they would be able to have."
Also worth noting, Hunt said, is that female mice that were exposed to intermediate level of BPA did not show the same reproductive decline as those exposed to both more and less of the chemical. That result may seem strange, but it's consistent with the way hormones -- and in turn, hormone disruptors -- work. At different doses, they can do very different things.
Understanding the nuances of BPA's effects at a variety of levels, including very tiny ones, will be essential as health organizations hash out safety limits for chemicals like these, Soto said.
"In another experiment that we published in 2006, we found alterations in behavior, and they happened at a dose that didn't produce reproductive effects in this study," she said. "If you are an agency that regulates the use of this chemical, it's not a good idea to just try a very high dose and extrapolate results from that dose."
Experiments with animals in her lab and others, Soto added, have linked very low levels of BPA with breast cancer, prostate cancer, obesity and more.
"All of these pathologies we've observed in animals perinatally exposed to BPA are on the rise in humans," she said. "For ethical reasons, comparable studies cannot be performed in humans. But what happens in animals is very likely to happen in humans."