Prenatal BPA Exposures (Don’t) Affect Birth Weight
By Steve Hentges | October 7th 2015 07:18 AM

http://www.science20.com/steve_hentges/prenatal_bpa_exposures_dont_affect_birth_weight-157393

Summary
Recent media stories have reported on two new scientific studies involving BPA’s effects onbirth weight. One study reported a statistical association between prenatal exposure to BPA
and increased birth weight, while the other reported an association with decreased birth weight.
However, both of the new studies use cross-sectional study designs that are inherently
incapable of providing much meaningful information on whether exposure to BPA has any effecton birth weight. Furthermore, the BPA exposure measurements reported in one of the studies
are not likely to be valid at all.
In comparison, multiple comprehensive studies on laboratory animals have found that exposure
to BPA has no effect on birth weight. The lesson to be learned (again) is that cross-sectional
epidemiology studies are unlikely to provide useful information on any potential health effects
from exposure to BPA.
Background
For 10 years or more, the popular media has repeatedly written about the chemical bisphenol
A(BPA). Particularly popular are stories about new scientific studies, along with the suggestionthat we’re all at risk of one health effect or another due to everyday exposure to BPA. In that
context, recent reports on prenatal exposure to BPA and birth weight were just more of thesame.
In scientific terms, more of the same might be called replication, which generally is a goodthing. The results of one study are more likely to be valid if the results are replicated in anotherstudy, in particular a study conducted independently by other researchers. In fact, one of thehallmarks of the scientific method is replication, or reproducibility of results.
New Studies and Results
But replication isn’t what happened a couple of weeks ago, although you wouldn’t have known
that from a quick review of media stories. What did happen is that two new studies werepublished in the scientific literature, about a week apart, on the subject of prenatal exposure toBPA and birth weight, and both studies were separately covered by the media. Along with birthweight, both studies measured BPA exposure at the time of birth and both studies reported anassociation between BPA exposure and birth weight.
The problem is that one study reported that prenatal exposure to BPA was statisticallyassociated with decreased birth weight while the other study reported an association
with increased birth weight. So which is it, does prenatal exposure to BPA increase or
decrease birth weight? Digging a little deeper into the study designs and some of the data
reveals that neither study is capable of answering the question.
A fundamental limitation of both studies is that these associations with birth weight were basedon cross-sectional study designs in which health outcome and exposure data are collected at asingle point in time. With cross-sectional study designs, it is simply not possible to know if theexposure preceded the health effect, which is a necessary element to establish causation.
The lack of a temporal sequence is a particular issue for BPA because it has a very short halflife in the body and it is well established that exposure measured at one point in time says very
little about exposure at preceding times. For this reason alone, neither study can speak to theeffect of prenatal exposure on birth weight.
Furthermore, the BPA exposure measurements in one of the studies are likely to be invalid. Itis well known that BPA is efficiently metabolized and quickly eliminated from the body inurine. Accordingly, urine is the “optimal matrix for measuring nonpersistent, semivolatile,hydrophilic agents,” such as BPA. That’s not just an uninformed opinion but represents theviews of a group of highly qualified experts led by a prominent scientist from the U.S. Centersfor Disease Control and Prevention (CDC), which is recognized as a worldwide authority onbiomonitoring.
In contrast, measurement of BPA in blood is not optimal due to significant challenges, including
the high demonstrated potential for sample contamination with BPA. The same group led by
CDC previously advised against using blood measurements, stating that “it is seldom possible
to verify that [blood] concentrations of [BPA] are valid measures of exposure.”
Recognizing the difficulty in measuring trace levels of BPA in blood, researchers from thePacific Northwest National Laboratory and the U.S. Food and Drug Administration (FDA)
recently published clear criteria to identify contaminated blood samples. For oral exposure toBPA, which is the primary way we contact BPA through our diets, blood samples

containing “>1% BPA [in unmetabolized form as a percentage of total measured BPA including
its metabolized forms] are likely contaminated.”
This makes sense since extensive research in human volunteers has shown that very little
unmetabolized BPA would ever be present in human blood. The levels of unmetabolized BPA
reported in the recent study are frequently above 90% (!), a clear indication of contamination
rather than a measure of exposure.
Does Prenatal BPA Exposure Affect Birth Weight?
Although the two recent studies provide essentially no meaningful information on this question,
we have a wealth of relevant information from comprehensive, high-quality studies in laboratory
animals. These studies provide extensive, replicated evidence that prenatal exposure to BPA
has no effect on birth weight, in particular at any level of exposure even remotely close to actual
human exposures.
What we should have learned from the two new studies, contrary to the headlines theygenerated, is that cross-sectional epidemiology studies on BPA are generally incapable ofproviding any useful information on whether exposure to BPA could cause health effects. Thatlesson is not new, but bears repeating based on recent history since misunderstanding of thevalue of cross-sectional studies certainly contributes to misinformation in the popular media.