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Exploring the relationship between weight, BMI, and fertility

The relationship between weight and fertility is a sensitive and complicated one. With so much weight bias in the medical industry, it can be tricky to figure out what to believe. While there is research that supports that weight affects fertility, all bodies are different — and an individualized approach to discovering what’s true for your body is critical. Women of all sizes have successful pregnancies and give birth to healthy children.

That said, knowledge is power. So, in an effort to navigate all of the information out there about how weight can impact fertility, we’re taking a look at what the science actually says.

  • Research indicates that weight can affect your fertility, but all bodies are different — there are many factors that impact your ability to conceive.
  • Hormones play a major role when it comes to fertility. Being "underweight" or "overweight" can throw off your hormones, which in turn can impact your periods and ovulation.  
  • You don’t necessarily need to be within the “normal" range for BMI (which is between 19-24) to increase your chances of getting pregnant.
  • If you don’t have a BMI within "normal" range and are trying to conceive, talk to a health professional who specializes in metabolic fertility issues.
  • Remember: BMI is not a perfect measurement — and many women with low and high BMI successfully get pregnant.

First things first: What is BMI?  

Body mass index, or BMI, is a way to measure weight relative to height and is often used as a way to predict body fat.

Your BMI is calculated by taking your weight in kg divided by height in meters squared (kg/m^2). You can use an online tool like this one to calculate your BMI, or your healthcare provider can help you.

Here are the general guidelines for women:

  • Under 19: "Underweight"
  • 19-24: "Normal"
  • 25-29: "Overweight"
  • Over 30: "Obese"

But, BMI is an imperfect measurement, and doesn’t necessarily speak to overall health. It can't distinguish between fat and muscle, which means that you could be toned and still register as having a high BMI, because muscle weighs more than fat.

BMI also can't tell the difference between various kinds of fat, or how fat is distributed in the body, which is an important distinction. Visceral fat, for example, is in the abdominal cavity and wraps around organs like the liver, pancreas, and intestines. High amounts of visceral fat can lead to health problems, such as Type 2 diabetes. People of all sizes can have higher-than-expected amounts of visceral fat, but a BMI test can't ascertain that.

“Although BMI has been the gold standard test for classifying patients as underweight, normal weight, overweight, or obese, it’s a poor test for measuring the health of the patient,” explains Marta Montenegro, a nutrition fertility lifestyles specialist at IVFMD. “I can’t say that patient ‘X’ is healthier than ‘Y’ solely based on BMI.”

How BMI and weight can impact your fertility

It's important to remember that many women with low and high BMI successfully get pregnant.

Conception depends on many factors, including the health and function of your reproductive system, as well as your hormone levels. An egg and sperm need to meet at just the right time — otherwise known as your fertile window — for fertilization to occur. Your hormones need to be at normal levels for all of this to take place. "Normal" depends on many factors, including your age and where you are in your menstrual cycle (i.e. follicular phase, luteal phase, etc).

Estrogens, specifically how much of the hormones you have, play a major role when it comes to weight and fertility. Estrogens are produced by the ovaries, as well as in fat cells and the adrenal gland.

If you have a high BMI, you may have more fat cells. The more fat cells you have, the more estrogens are being produced in your body. Estrogens are also responsible for the growth of the uterine lining, so more fat cells lead to a thicker uterine lining, and heavier periods. Enough estrogens can interrupt your cycle, lead to infrequent periods, and prevent you from ovulating. High levels of estrogens can also affect how your body processes insulin, therefore causing problems like diabetes and insulin resistance, as is the case with polycystic ovarian syndrome, or PCOS (more on this below). According to the American Society for Reproductive Medicine (ASRM), a higher body-fat percentage can also affect the success of fertility treatments, such as IVF.

On the flip side, when you have a low BMI, or a low body-fat percentage, your body doesn't have enough energy to run properly. When this happens, non-essential systems, like reproductive function, slow and shut down. Your hypothalamus, where hormones such as estrogens are regulated, gets sluggish, and this leads to irregular, or entirely absent, menstrual cycles. "Irregular" could mean a shorter time between periods, a period lasting longer than eight days, or skipping cycles altogether (lack of a menstrual cycle is known as anovulation).

Certain health conditions can also affect BMI

Despite what our mainstream culture would have us believe, being "overweight" or "underweight" isn’t just about how many calories you’re taking in. For example, research has been looking into how genetics plays a role in your body weight. There are also treatable medical conditions, like hormonal conditions or eating disorders, that play a role in BMI.

Polycystic ovary syndrome (PCOS) is a hormonal condition that can cause irregular menstrual cycles, extra hair growth, fatigue, and weight gain. If you have PCOS, you’ll often have much higher levels of anti-mullerian hormone (AMH). You may also have higher levels of androgens (called hyperandrogenism) — but not all women with PCOS experience this. Androgens are a class of steroid hormones often thought of as "male" (including testosterone, androstenedione, and DHEAS), but are actually present in all healthy adult bodies. In women, they’re produced in the ovaries and adrenal glands.

Insulin, the hormone that allows your body to absorb blood sugar, is also part of the PCOS equation — if you have PCOS, your body may not be as responsive to insulin as it's supposed to be (aka "insulin resistance"), so your blood sugar doesn't get processed as easily. This may result in chronically elevated blood sugar and insulin levels. 80% of women with PCOS have a higher than normal BMI due to the fact that it's a metabolic disorder, in which the body’s metabolism fails to regulate a specific substance the body needs (in this case insulin), and how it interacts with glucose in the body. Women who are "overweight" can have more issues with insulin resistance than those who aren’t.

Eating disorders, such as anorexia, can also lead to nutrient deficiencies that decrease levels of estrogens, negatively affecting ovulation and leading to the absence of periods altogether. When a woman is in recovery, and gaining weight, she might also experience changes in her cycle as her hormones recalibrate.

What to do if your BMI isn't within the "normal" range

You don’t have to be at a BMI over 18 and under 25 to increase your chances of getting pregnant. But if you’re classified as "overweight" with a high BMI, losing just 5-10% of your body weight may positively impact your fertility. A 2014 study showed that people who were "overweight" and being treated for infertility lost 10% of their body weight and had higher rates of conceiving than those who didn’t. (Note: There are also meta-analyses — studies that combine estimates from a whole bunch of studies — that also suggest that weight loss has a positive effect on fertility.)  

Losing weight, however, isn't necessarily the answer. Dr. Montenegro recommends talking to a health professional who specializes in metabolic fertility issues. Without being properly evaluated, you may be doing more harm than good for your fertility by losing subcutaneous body fat — the fat you can “pinch” — and not losing visceral body fat, the latter which is the root of metabolic fertility issues, according to Montenegro.

Being "underweight" or "overweight" can have varying complications for your fertility. The good news is that there are steps you can take to improve your overall health, especially when it comes to conceiving. One of the things you can be proactive about is getting your hormone levels tested with Modern Fertility. Knowing your levels can help start a conversation with your doctor about your ability to conceive, and they can work with you to decide whether your BMI could potentially be a factor in getting pregnant.

This article was medically reviewed by Dr. Nataki Douglas, the Chair of the Modern Fertility Medical Advisory Board.

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Risa Kerslake

Risa Kerslake is a registered nurse turned freelance writer. Her work has been featured in THINX, Healthline, Broadly, and Today's Parent. You can find her at RisaKerslakeWrites.com and @risakerslake

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