Junkfood Science: Trying to have a baby?

December 15, 2007

Trying to have a baby?

For most of human history, fat has been life-sustaining — seen as security against scarcity, and a sign of fertility in women and of their ability to bear and nurture children. Nowadays, every month seems to bring another scare about women’s fat. This week, headlines warned women that being fat hurts their chances of conceiving. It undoubtedly worried countless women. It needn’t have. The study behind the headlines didn’t support such a warning at all.

As Reuters reported:

Obesity hurts a woman's chances of conception

Obesity decreases the chances that a woman will get pregnant, and the more obese she is, the worse her prospects of conception, Dutch researchers said on Tuesday. Researchers at the Academic Medical Center in Amsterdam looked at how obesity affects women who are still ovulating but having trouble with conception. The study, published in the journal Human Reproduction, included more than 3,000 couples between 2002 and 2004 in 24 hospitals in the Netherlands.

Dr. Jan Willem van der Steeg and colleagues looked at the relationship between fertility in these women and their body mass index, a ratio of weight to height. ...

Already, this tells us that this was an epidemiological study looking only for preliminary correlations among a large database, of 3,029 couples to be exact. Reading the study itself reveals even more precautions before we react to its findings. The most significant methodological flaws in this study were its failures to include some of the most important confounding factors for infertility or to demonstrate tenable correlations, and its sample bias.

Study population

The database they used was from the Collaborative Effort for Clinical Evaluation in Reproductive Medicine (CECERM) cohort of couples referred for infertility work-ups in the Netherlands betweeen January 2002 and February 2004. These women had regular menstrual cycles and patent fallopian tubes, and their male partners had good motile sperm counts. For this computer analysis, the authors included the couples’ heights and weights and smoking habits. They had complete follow-up information on 2,793 couples. Of those, 1,1,36 began fertility treatments. Within one year, among the 1,651 without fertility treatments, 529 spontaneously became pregnant and 1,060 didn’t. The researchers then looked for correlations between BMIs and spontaneous pregnancy during the first 12 months.

The couples in this cohort were generally older than typical new parents. The average age of the women was 32 years, ranging from 25-39; and the men averaged 35 years of age, ranging from 27 to 44. We don’t know the break down of the ages of the women and men relative to conception. Yet, age is one of the most significant factors that diminish fertility in women, as well as in men.

The average BMI of the women was 22.9, ranging from 19 to 33. But the researchers did not look at recent weight changes or dieting behavior and, as we know, nutritional status and fat intakes play a role in fertility. The BMIs of the prospective fathers were also not reported. Most significantly, they did not break down the BMIs by age, and BMIs would be expected to rise with age.

Untenable associations

They reported that their computer model adjusted for the women’s age, previous pregnancy and duration of subfertility, semen motility, referral status, and smoking status of both partners. Compared to women with BMIs of 21-28, they found that each unit BMI ≥29 was associated with a mere 4% lower odds ratio for spontaneous pregnancy within one year, and each BMI <21 was associated with a 5% lower odds ratio. Concerning the significance of these findings, they stated:

The spline analysis showed that BMI had an inversed U-shaped relationship with the probability of pregnancy, although this was not statistically significant over the whole range

Their modeling also found age associated with a 6% lower odds of getting pregnant for every year over 30. In contrast, current smoking in the women was associated with a 20% improved odds of getting pregnant.

The factor associated with the greatest risks for not getting pregnancy — odds 33% lower — was referral from third care clinics. It is odd that these researchers chose to include these couples in this risk analysis, because in their paper earlier this year, where they developed their model for predicting spontaneous pregnancy in subfertile couples using this same CECERM cohort, they specifically stated that they excluded couples referred from these tertiary fertility clinics because these couples had had prolonged fertility work-ups (which included an HSG or laparoscopy) and were older and had medical issues indicating the need for assisted reproductive treatments, which would skew the results.

Missing confounding factors

Attributing the untenable odds ratios to being the fault of obesity itself is especially unsound given the limited data and quality of the evidence, and paltry attempts to rule out known confounding factors. For instance, we don’t know the couples’ past or current medical treatments and medications, whether either partner had STDs or venereal diseases, if the women had pelvic inflammatory disease or endometrial cysts, or if they drank (drinking alcohol is associated with becoming pregnant more quickly). And the most glaring missing confounders: socioeconomic status and employment situations. Stressed, overworked couples and an inability to get the necessary frequencies of intimacy, greatly reduce chances of getting pregnant. The heavier women would be most likely to have been dieting and restricting dietary fats, to be older, to be underemployed and to be dealing with social stresses — all factors which could contribute to delayed conception regardless of weight.

Even these authors admitted to some of these limitations stating: “A limitation of this study is that frequency of intercourse was not taken into account... in view of the paucity of data, confounding factors like medication and adverse lifestyles could not be ruled out.”


But did you catch the biggest flaw of all? Here’s a hint: Did they look at all women of childbearing age and the relationship between BMI and getting pregnancy? No. This study cohort is a very selective population of women with fertility problems. The results cannot be generalized to all fat women. So, to conclude that being fat hurts women’s chances of getting pregnant isn’t correct. And this isn’t just a speculative example.

In February, researchers at the National Institute of Environmental Health Sciences, National Institutes of Health, Department of Health and Human Services, Research Triangle Park, NC, published an analysis of obesity and how long it took women to get pregnant. They examined data on 7,327 women in the United States and found an untenable 18% odds ratio between obesity (BMI >30) and lower fecundability (probability of conceiving in a given menstrual cycle) among women trying to get pregnant. [This translated to it taking 2 months longer for an obese woman to become pregnant compared to those of ‘optimal’ weights. They cautioned, however, that “the association between obesity and fecundity may be weaker than we observed” because their cohort included women with polycystic ovary syndrome (PCOS). While it is commonly ‘associated’ with obesity, obesity is a symptom of PCOS and not a consequence of obesity.] But when these researchers compared women who conceived in planned pregnancies with those who had unplanned pregnancies, they found no difference with BMIs.

Putting things into context

To begin to make sense of all this, as an observational study looking for statistical correlations, we know we can’t credibly make any assumptions about cause. Add to that the untenable correlations, missing confounding factors and unrepresentative study population, and this preliminary study doesn’t begin to support the overreaching conclusions in the headlines.

And how does this study fit into the body of research to date and build upon other evidence? It doesn’t. And it goes beyond contrasting with human history which has long associated fat women with fertility.

As these author’s state: “Up till now, the relationship between BMI and pregnancy chances had not been established in ovulatory subfertile women. This is the first prospective cohort study to demonstrate this.” In other words, this has never been shown before.

As the old science adage goes: Extraordinary claims require extraordinary proof. This study did the former and flopped on the latter.

More accurate reporting of this study might have said: Among older couples having difficulty conceiving, no tenable correlation was found between obesitiy and the chances of getting pregnant.

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