Weight and Fertility: What Does the Science Say?
(Note on language - I use “fat” as a neutral descriptor as it is used by the fat activism community. I also don’t use the terms “ob*se” and “overw*ight” because these terms are highly stigmatizing, were created to pathologize fat bodies, and are rooted in racism and, specifically, anti-Blackness. To learn more, read Fearing the Black Body - The Racial Origins of Fat Phobia by Sabrina Strings and Belly of the Beast - The Politics of Anti-Fatness as Anti-Blackness by Da’Shaun Harrison.)
It’s almost impossible to receive medical care, especially fertility or pregnancy care, without hearing all about weight. And polycystic ovary syndrome (PCOS) is a common condition that impacts both weight and fertility. Higher body weight is frequently described as a “cause” of infertility, but this claim deserves a lot, and I mean a LOT, of scrutiny, as the research base supporting these claims is highly problematic. The reality is that fat women have been having babies throughout human history and women of any weight can struggle with fertility, miscarriage, or health issues. The science on weight and fertility, like with health in general, is tricky and usually not as it appears. Let’s dig into it.
But, before we get into the impact of a higher weight, it is important to know that caloric restriction and excessive exercise can both cause infertility. Irregular periods are often listed as a symptom of eating disorders. Our bodies are smart and know when we are not getting enough food to sustain a pregnancy. Also, proper fuel and nutrition are vital throughout pregnancy. Trying to lose weight while trying to conceive or doing a pregnancy has significant risks.
TLDR: the research showing higher body weight “causes” infertility is highly problematic and only shows a correlation, which is likely explained by factors that researchers didn’t bother to examine. Even if weight did cause infertility, pursuing weight loss is not an evidence based intervention, as the overwhelming evidence shows that weight loss attempts cause weight gain and increases risk for physical and mental health problems. Fat bodies are capable of healthy pregnancies.
It is true that higher weights are correlated with infertility, but… correlation does not equal causation. Ice cream sales are also correlated with the murder rate, but you don’t see people arguing that ice cream causes people to kill each other (unless your partner finishes your ice cream without permission 😉). In most cases, it is an unidentified third variable that explains the correlation (in this example, summer weather explains both increased murder rates and ice cream sales).
Most of the research linking higher body weight to infertility does not control for PCOS, which causes both weight gain and problems ovulating. This is really important to note because PCOS patients are often told incorrectly that their weight causes PCOS and infertility, when really it is the PCOS causing both.
When I say “does not control for",” what this means is that the researchers didn’t account for how much of the relationship between weight and fertility is actually explained by PCOS. In the ice cream example, the correlation between ice cream sales and the murder rate did not account for how much impact summer weather has- if it had, it likely would not have found a relationship between ice cream and crime.
The few studies that do control for PCOS are problematic in their own right, as they do not control for the impact of weight stigma or weight cycling (repeatedly losing and gaining weight, AKA “yo-yo dieting). This is critically important because both of these factors independently predict the health problems that are typically blamed on fatness, regardless of weight. What this means is that we have quality scientific evidence that experiencing stigma and shaming about weight (including weight bias in healthcare providers) and weight cycling are independent risk factors for a ton of health problems (diabetes, hypertension, etc.), no matter what a person weighs, yet nobody doing fertility research has bothered to examine if these two factors could explain the link between higher weights and fertility. This is inexcusable and it is an example of systemic anti-fat bias in research.
Further, even if there was quality evidence showing that higher weight contributed to infertility, there still is no weight loss intervention that has quality evidence showing people can maintain weight loss in the long term. Again, pursuing weight loss is not an evidence based intervention- there is overwhelming evidence showing that pursuit of weight loss does not work in the long term and causes weight gain, as well as weight stigma, and weight cycling (both of which are by themselves risk factors for a variety of physical and mental health problems). And yes, this includes all medication, all diets, and all bariatric surgeries.
And we know that restricting calories and excessive exercise are known contributors to infertility problems.
The available research on this subject has either found that losing weight had no effect on fertility, or they found an impact but did not control for the impact of health behaviors. What I mean by this is that when people pursue weight loss, they often (but not always) adopt healthy behaviors, like exercise and eating vegetables, that lead to improvements in health, regardless of weight or weight lost . But researchers generally don’t bother to see if it was these behaviors that improved health or the weight loss, they just see that weight loss was correlated with improvements in health and, voilá, conclude that weight loss improves health.
What is especially maddening is that many of these studies find that there was no significant impact of weight loss, yet in the very next paragraph go on to recommend it despite their own findings (i.e. Best, Avenell, & Battacharya, 2017)! Often times the titles and abstracts of these studies support weight loss as an intervention when when the data of the study itself doesn’t support it and then these snippets get picked up and blasted all over the media. This another example of systemic anti-fat bias in medical research and the media.
When researchers do separate health behaviors from weight, they find that when people engage in healthy behaviors (eating more than 5 servings of produce daily, exercising at least 12 times a month, drinking below recommended limits, and not smoking) there are no meaningful differences in the mortality of “normal” weight, ob*ese, and overw*ight people (e.g. , Gaesser & Angadi, 2021; Matheson, King, & Everett, 2012). All three groups benefited from these behaviors as well. So it is reasonable to conclude that any benefits to fertility were likely explained by adoption of these health behaviors, rather than (temporary) weight loss, and we definitely cannot definitively conclude that weight loss alone improved fertility unless health behaviors are controlled for.
And then there’s this, as Nicola Salmon and Ragen Chastain point out in their excellent review of this research: doctors often say (without quality evidence) that losing 5-10% of weight, or even 5-20 pounds, can boost fertility, but yet they say that BMI impacts fertility and folks in higher weight bodies could lose this required amount of weight with little to no change in BMI and still be deemed “unhealthy.” Make it make sense!
In conclusion, even if we could definitely say that higher weight causes infertility (which the research most certainly does not show), pursuing weight loss is not an intervention that is supported by the research and it has risks to fertility (weight cycling, weight stigma, the negative impact of restriction on fertility).
For more information on weight and fertility, check out the work of Nicola Salmon (fat positive fertility expert) and Ragen Chastain, fat activist, the author of Weight and Healthcare, and consultant to help healthcare providers unlearn weight bias. Here is their article that summarizes the research and contains additional resources and a long list of references for the scientific studies they dug into. Also check out the Reproductive Health section of my post on resources to improve body image and fight weight stigma.
For more information on fertility treatment, check out my post Everything You Need to Know About Infertility and Assisted Reproductive Technology, as well as How Infertility Affects Mental Health.
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About the Author, A Los Angeles Infertility Therapist:
Dr. Linda Baggett is a Licensed Psychologist at Well Woman Psychology, serving clients online in California, Illinois, New York, and Washington. She received her PhD in Counseling Psychology from the University of Memphis. As infertility therapist in Los Angeles, she specializes in supporting women through their journey to hopefully have a child, no matter when the end result. With a focus on women’s issues, she also has expertise in relationship issues, sexuality, pregnancy loss and miscarriage, birth trauma and postpartum issues, trauma and PTSD, EMDR, and body image and size-based oppression.
Disclaimer:
This blog is for educational and informational purposes only, is not a substitute for individual medical or mental health advice, and does not constitute a client-therapist relationship.