As slogans go, the BBC News’s “Floss for Fertility” was definitely the catchiest. It was certainly more pleasing to the ear than alternatives such as “Brush Regularly to Get Pregnant” (as appeared in the Times of India) or “Women Who Want to Get Pregnant Need to Clean Their Teeth” (from the Daily Mirror). These toothy sound-bites featured in news stories published over the last few days, and refer to research presented at a major international conference on human reproduction that took place this week in Stockholm. The researchers reported a link between periodontal (i.e., gum) disease and pregnancy outcomes, which they derived from a large-scale study of over 3,400 Australian women. We already know that the inflammation caused by gum disease can create tissue-damaging events capable of passing into the circulation and contributing to conditions such as diabetes and heart disease. The Australian team now suggest that such effects might also lead to a negative influence on fertility.
So, is it true that minding your choppers will affect your likelihood of conception? Well, maybe yes. But not perhaps in quite the way that this week’s media reports might have you believe.
The first thing to point out is that several media reports were wrong to imply that the research looked at a woman’s chance of getting pregnant per se. In fact, all of the women in the study successfully conceived, whether or not they had gum disease. In other words, all of the women were fertile, whether or not they flossed.
The real focus of the research was not on whether pregnancy occurred, but on how long it took to occur. According to the investigators, women with gum disease actually conceived okay. It’s just that it took them two months longer on average to do so compared to women who didn’t have gum disease. In this context, headlines like “Women Who Want to Get Pregnant Need to Clean Their Teeth” or “Healthy Teeth Equals Pregnancy” are perhaps a little misleading.
The second interesting thing to note is that the study underlying these findings was not designed to test this particular research question. In fact, it is not actually a new study. The project (officially nicknamed The Smile Study) was initiated in 2005 and its results were published in 2009. Its focus was not on fertility at all, but instead on preterm births and related obstetric complications; specifically, it aimed to see if the treatment of periodontal disease would have a positive impact on limiting premature births. As it turned out, the data did not support the researchers’ predictions as dental treatment was found to have no discernible beneficial effect on pregnancy outcomes. In fact, the best the researchers could conclude was that such dental treatment was (merely) not hazardous when used with pregnant women.
So how come this study re-emerged in 2011 at a conference on human fertility? Well, it appears that the researchers decided to conduct some data-mining on their old dataset in order to look at some new — and peripheral — hypotheses. Specifically, they found that: (a) of the 3,400 women screened for the study, around 26% had gum disease; and (b) these women with gum disease took an average of seven months to conceive, compared to an average of five months for women whose gums were healthy.
Now I don’t know about you, but I think this is a very small difference. Furthermore, these analyses are not yet fully reported (or peer-reviewed) and so we can’t draw too many conclusions regarding their soundness. All we have to go on is the conference press release. One problem is in discerning the precise nature of the statistical effect. We can note that, although the averages for both groups are reported, we don’t know what the ranges of conception-delays are. It could be that some of the women with gum disease took just five-and-a-half months to conceive, or that some of the women without gum disease took six-and-a-half months.
Whatever the ranges, we can probably assume that the difference in averages is statistically significant. However, it’s just not that big. An average of an 8-week delay in conception is not going to be disruptive to most people’s family planning arrangements. For example, it is unlikely to prevent many couples from achieving the number of children they would wish to produce in their lifetimes.
In addition, while the researchers might be lauded for their ingenuity in extracting new information from old data, it needs to be borne in mind that the dataset itself is not optimally set up for this type of hypothesis-test. In fact, it is essentially a retrospective observational study, not a randomized controlled trial (comparing flossers with non-flossers), nor a case-control study (comparing — and I apologise in advance for this — fertilizers with non-fertilizers). As such, any number of other variables may be responsible for the observed statistical association.
For example, before interpreting the association between gum disease and conception time, it will be important that overall health or hygiene is accounted for. People with gum disease may also be in poor general health, have poor health habits (such as smoking, drinking, or a bad diet), or be generally unconscientious or lazy. Any one of these might help explain why these women took (slightly) longer to conceive. Similarly, frequency of sexual intercourse will be important. After all, perhaps there is a link between poor dental hygiene (and bad breath) and frequency of sex? If so, then we might reasonably expect people with gum disease to reproduce just that little bit more slowly.
We need also to remember that much of what is important in this study will have been measured using self-report methods. Therefore, it might be difficult to rely on the accuracy of information regarding conception attempts, sexual activity, and the like, especially if poor dental hygiene is in any way related to poor personal self-discipline.
Interestingly, the researchers (and many media commentators) were quick to conclude that, because they believe they have established a causal link between gum health and adverse pregnancy outcomes, this same information can be used to propose an appropriate solution. In other words, they suggest that…
…all women should also be encouraged to see their dentist to have any gum disease treated before trying to conceive. It is easily treated, usually involving no more than four dental visits.
Now I am always a little skeptical about claims that epidemiological findings (i.e., “What caused it?”) can be used to support clinical applications (i.e., “What fixes it?”). Things don’t always work that way.
And we should be especially wary of such recommendations if there exists a possible conflict of interest. In the present case, while the study was funded by a project grant from the Research Council of Australia, as well as by a number of charitable agencies, we need also to note the source of some private monies that were donated. Specifically, the Colgate-Palmolive Company contributed financially toward the salaries of the dental hygienists involved in the study. Furthermore, the dental care manufacturer Oral B — who specialize in dental floss, toothpaste, and electric toothbrushes — supplied the hygiene products given to participants as part of the study protocol.
So two major manufacturers of dental hygiene products provided financial support to a study that eventually leads to a press release promoting the (fringe) benefits of using…dental hygiene products.
Unfortunately, neither the official press release nor any of the press stories I came across mentioned this corporate sponsorship link.
In the end, “Floss for Fertility!” may prove more useful as a marketing slogan than as a health promotion one.
Brian Hughes is an academic psychologist and university professor in Galway, Ireland, specialising in stress, health, and the application of psychology to social issues. He writes widely on the psychology of empiricism and of empirically disputable claims, especially as they pertain to science, health, medicine, and politics.