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The Hazards of Using BMI to Define Your Pregnancy

One of the most common questions pregnant people ask their provider is, “How much weight should I gain?” followed by, “How do I avoid gaining too much weight when pregnant?” Weight is an emotionally-loaded topic for many pregnant people, and providers know this. As a result, many providers struggle to talk with patients about healthy pregnancy weight gain, pregnancy risks associated with plus-sized pregnancies, or losing weight to improve fertility.

So, providers turn to body mass index (BMI) to avoid upsetting their patients and to standardize their approach to pregnancy weight gain and body size discussions. But is BMI really an accurate measure of how healthy a pregnant person is, the likelihood of a healthy pregnancy, or the best guide for a pregnancy care plan? Emphatically,  “No!” would be the answer from more and more providers, pregnant people, researchers, and advocates from the Health At Every Size (HAES) movement. Learn more about how the maternity care system relies too heavily on BMI and how you can avoid letting your BMI define your pregnancy.

Body mass index (BMI) is a calculation using height and weight to evaluate your body size. Healthcare providers use BMI to assess body fat. It is an indirect measurement only. BMI is calculated by dividing your weight by your height squared: BMI = weight (kg) / height (cm2). It’s not a direct measurement, such as scanning your body with an X-ray, MRI, or calipers.

In 1998, at the height of the low-fat diet craze, the National Institutes of Health approved BMI as the standard measurement of health. With that, BMI officially became a health indicator, used to put people into underweight, ideal body weight, overweight and obese categories.

BMI is the default measure for obesity because it is the easiest and cheapest way to measure body fat. As a result, healthcare providers can easily put people into risk categories using the same measurement across a range of patients. In addition, researchers need BMI to track population-wide weight trends.

For example, researchers can look at the BMIs of many pregnant people to try to find associations between pregnancy outcomes and certain BMIs. Some research studies show that women with higher BMIs are more likely to have gestational diabetes, hypertensive disorders of pregnancy, or larger babies. Your provider then generalizes the results of these studies of thousands of women’s BMIs to your BMI and prescribes the recommended amount of “safe weight gain” for you in pregnancy.

But does your BMI accurately consider all your health conditions or risk factors? For example, how much do you exercise? What type of diet do you follow? What is your family history of high blood pressure or diabetes?

The Belgian mathematician Adolphe Quetelet developed BMI scales in 1832 using data from predominantly European men to measure weight in different populations. Why are medical professionals still using charts based on white men of European origin from almost 200 years ago to decide the ideal weight you can gain during pregnancy or what kind of testing you should have? Good question. There are many other problems and critiques of using BMI as a health indicator during pregnancy, such as:

  • BMI can not accurately be calculated once you are pregnant. So providers use pre-pregnancy BMI – typically calculated at your first OBGYN appointment. But your weight and body composition may have already changed at this point, especially if you did not find out immediately that you were pregnant or are struggling with hyperemesis and have lost weight due to constant nausea and vomiting.
  • At the same BMI, women have, on average, more body fat than men, and Asians have more body fat than whites. BMI scales do not take into account race or sex.
  • BMI is imperfect; it does not distinguish between body fat and lean body mass. Muscle is denser than fat and weighs about 18 percent more. So a person could have a lean body mass (very little excess fat) but weigh more because they have lots of muscle. Their heavier body weight would skew their BMI, putting them in an “overweight” and higher-risk pregnancy range, even though their pregnancy could be perfectly healthy.
  • People gain weight in different ways throughout their pregnancy. The recommended pregnancy weight gain categories based on BMI do not reflect this.
  • BMI does not directly correlate with health (someone can have a high BMI and be healthier than a person with a lower BMI, for example). Recent studies of BMI and health indicators (blood sugar, cholesterol, and blood pressure) found that more than half of people considered overweight by BMI were healthy by these other indicators.
  • The location of your body fat may matter more to your health than how much body fat you have when it comes to health risks.
  • The psychological damage caused by a pregnancy care provider telling a pregnant person they are overweight, obese, or have a high risk can cause physical and emotional stress and harm.

According to the American College of Obstetricians and Gynecologists (ACOG), there are significant risks associated with obesity and pregnancy. But how the provider talks about these risks with their plus-sized patients can be devastating, confusing, and downright harmful. In addition, many critics argue that continuing to use BMI to define a “healthy body” is racist and sexist because it is based solely on a white male standard. A standard that does not reflect the current reality that more than half of women in the U.S. begin pregnancy classified as overweight or obese based on BMI.

Using BMI in maternity care also perpetuates weight bias among maternity care providers, the health care system, and society. Weight bias can be unconscious or conscious. The current mindset of most care providers is women who are outside the normal BMI range aren’t a healthy weight and should be treated as though they are high risk. With the high-risk designation, pregnant people often have more appointments and testing. More appointments and testing add a time and financial burden to pregnant people. And all of this oversight sometimes leads to medical interventions that aren’t always needed.

Being classified as a high-risk pregnancy can limit a person’s access to midwifery care, desired birth location, and type of birthing experience. While care providers set their own guidelines, the American College of Gynecologists and Obstetricians states that obesity is not an indication for the transfer of routine obstetric or gynecologic care. Birth centers and medical facilities may have BMI restrictions for the size of people who can deliver at the facility. Current maternity care deserts in more rural areas of the United States mean that plus-sized pregnant women may have to travel longer distances and have limited choices for where, when, and how they deliver.

Being turned away from a selected medical facility can be stigmatizing and even traumatic for people. The bias baked into BMI (and using BMI to classify a pregnancy as high risk or low risk) contaminates the care provided. Most patients with an elevated BMI already report feeling stigmatized by healthcare providers.  This stigmatization leads patients with elevated BMIs to avoid care because they feel traumatized at each OB visit. An example of the traumatization some plus-sized people experience is being forced to weigh themselves at every prenatal visit and repeatedly discuss their pregnancy weight gain with multiple nurses and providers.

Critics of BMI and weight-biased maternity care argue that there is a better way to care for pregnant people. Researcher and author of Health At Every Size writes, “well-being and healthy habits are far more important than any number on the scale. Many maternity care providers and members of the anti-fat bias movement support applying the HAES model to pregnancy care.

Providers can help empower pregnant people from a strength-based perspective, supporting what people are already doing to stay healthy and feel good. This approach removes the stigma and avoids making people feel “broken” before they even start their pregnancy. It is unlikely that a pregnant person with a larger-sized body has never been told before that they are overweight or obese, and they have no doubt already tried countless diets or ways to lose weight. Many patients with an elevated BMI prefer not to use the word “obese” or “obesity.” You should feel comfortable asking your maternity care provider or other office staff to not use this terminology.

Unfortunately, there is no directory of size-friendly or HAES-certified maternity care providers yet. Until there is, here are some questions you could ask as you select your midwife or ob-gyn to help you find a more size-inclusive provider:

  1. Do I have to be weighed at every prenatal visit? What other arrangements have you made for prior plus-sized patients?
  2. If I have an elevated BMI, what does this mean for my pregnancy in terms of testing and prenatal visits during my pregnancy, my delivery plan, or care overall?
  3. Have you heard of Health of Every Size (HAES)?
  4. How do you support your pregnant patients with a history of disordered eating or are in larger-sized bodies that might be labeled as high risk?

Asking these questions will help you find a healthcare provider who will promote your individual health based on your individual needs. Your specific body has specific needs, especially during pregnancy. Talking to friends, colleagues, or family members about their weight-related pregnancy experiences will also give you an idea of more body-inclusive maternity care providers.

While the burden of teaching your doctor or nurse-midwife how to talk about weight and health or the HAES approach to pregnancy care should not fall on you, there are some ways for you to reduce some of the health hazards of an unhealthy focus on BMI. For example, working with a size-friendly nutritionist or dietician during pregnancy can help you navigate the challenges of eating well during pregnancy in any size body. Likewise, requesting a referral from your obstetrical provider to a size-friendly physical therapist can help you find ways to stay active and move your body injury-free through your pregnancy.

Continuing to use BMI as a standard part of pregnancy care only perpetuates the use of painful and harmful labels for plus-sized pregnant people. Change in the healthcare world does not come quickly and easily. While the curriculum in medical and nursing schools may be slowly changing the way tomorrow’s providers care for pregnant people of all shapes and sizes, there are some steps you can take to limit how much BMI defines your pregnancy. An essential part of having a positive pregnancy and birth experience is being cared for by a provider you can trust who makes you feel understood. If your provider’s weight bias or preoccupation with BMI does not make you feel safe or well-cared for, then you and your baby deserve better care.

Amy Harris
Amy Harris is a certified nurse-midwife with a Master's Degree in Maternal and Child Health from Harvard Chan School of Public Health. Her passions are health literacy and women's reproductive health. A recent two-year sabbatical with her family in Spain was the impetus for becoming a freelance women's health writer. An exercise nut, she is happiest outdoors and on adventures abroad.

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