Bodyweight (BMI), Fertility & Egg Freezing: What the Science Says
If you have significant struggles with your weight (on the very top or the lowest side of the scale) you may want to take longer to make some changes before your egg freezing cycle, as all these things are established fertility foes. It’s important not to try to make drastic lifestyle changes overnight, as this can put your body under a great deal of stress. For example, if you are severely overweight or underweight, a 20% rapid gain or loss in the opposite direction could cause menstrual irregularities and cause a hormonal imbalance. Taking your time to make healthy and steady lifestyle changes that are sustainable over time is by far your best strategy.
If you’re on the heavier side
If you are above average weight, you might consider losing some body fat prior to your egg freezing cycle in an effort to optimize your outcome. We are big supporters of body positivity, so this is not about body shaming or saying we’ve all got to look one uniform way at all. Culturally, artistically and romantically, there’s no right or wrong. However, scientifically, we’re on different ground.
Science is an out-and-out body shamer, and it is pointing the finger and saying that, in general, women who are heavier than a body mass index (BMI) of 25 have more fertility issues. By heavier, doctors usually use the terms “overweight” (classified as a BMI of 25–29.9) and “obese” (BMI of 30 and above.) There are plenty of free online BMI measurement tools that can help see if you fall into a fertility-friendly bodyweight but here is a quick equation below.
BMI is a notoriously tricky thing, of course. That’s why spreadsheets and data tables are great on the one hand, but on the other hand they really have their limits. They’re a little...reductive. When you just look at a number, there’s no context and no subtlety. We all know that you can be bigger or wear a plus size and still be perfectly healthy, especially if you're eating great things and working out.
So, when scientists say the optimal weight for fertility that they’ve identified is a body mass index (BMI) of 20-25, you’ve of course got to use some intelligent judgment. If you’re an athlete, or you have always just been super strong, or you know you live a healthy lifestyle but still that number comes out weird, then this probably won’t be a real concern - discuss it with your doctor. However, if you know your diet and exercise choices aren’t ideal and you’re carrying more body fat than lean muscle, this is the time to get activated.
What are the fertility issues associated with being overweight?
Research suggests that overweight and obese women are much more likely to experience anovulation, menstrual disorders, infertility, difficulties in assisted reproduction, miscarriage, and adverse pregnancy outcomes.
Even with the help of hormones used in the egg freezing process, most studies show that obese women going through IVF tend to have fewer eggs retrieved, fewer mature eggs, and poorer egg quality compared to women who are not overweight or obese. In fact, being obese is directly considered a risk factor for infertility.
One culprit for these infertility issues is rooted in the fact that your fat cells produce estrogen in addition to the estrogen already being produced in your ovaries. As such, the more fat cells you have, the more your body creates unnecessary amounts of estrogen. This excess estrogen has a domino effect on other hormones, throwing them all out of balance, causing reproductive issues such as irregular ovulation.
Being overweight also influences other factors that are problematic for reproduction, including stress, lack of sleep, insulin resistance, oxidative stress and more. For those with a more substantial percentage of weight to lose in order to get to a BMI below 25, the good news is that even a five percent reduction in weight has been shown to improve fertility.
Losing a little bit of weight can also help lower your risk of complications while under anesthesia. It’s better to aim for a smaller, steadier weight loss, even if you don’t immediately move into this “optimal” BMI range. It’s not just about egg freezing - the healthy lifestyle changes that lead to weight loss will have an immediate impact on long-term fertility and your overall well being – meaning you don’t have to cross the finish line into this BMI range before seeing improvements in egg quantity and quality.
If you’re on the slimmer side
People kind of laugh at the term “skinny shaming” as if it doesn’t exist and it’s not a real and hurtful form of body policing. Well, the optimal fertility scale is back, and it’s running its judgy eye over slimmer women, too. That’s to say, the data suggests that having a BMI of 20 or less also comes with a higher risk of infertility.
What are the fertility issues associated with being underweight?
This happens in part because key sex hormones like estrogen are produced and stored in fat cells. So, just as heavier women produce excess estrogen, skinnier women, who have less of what’s been coined “sex fat” (tissue where sex hormones are stored) can have lower estrogen levels.
Being very slim can also inhibit the production of other key fertility hormones such as leptin, which is thought to give ovaries the green light to function. This is most likely because leptin plays a role in the regulation of energy balance (how many calories you eat, burn and store as fat) and is produced and stored in fat cells. So, when fat cells are in short supply, leptin signals to your body that the conditions are inadequate for reproduction.
Why would your body demand some body fat in order to prepare itself for reproduction? Well, in part because pregnancy and breastfeeding each have high energy costs and place extra daily caloric demands on the body. So when calories are in short supply or you don’t have any spare body fat, it makes sense that your body prioritizes its day-to-day living-and-breathing functions over reproductive ones, knowing that an even bigger energy demand could be the result of getting pregnant.
Just as for women who come out on the heavier end of BMI measurements, these indices can be a tricky thing to navigate as some women are naturally very slender, have little body fat or struggle to gain weight. However, if you have low body fat due to a restrictive diet or excessive exercise, that could be a red flag for impaired fertility.
Although a BMI of over 20 is considered optimal for fertility - the Goldilocks zone - your doctor would probably only raise the topic with you if your BMI is under 18, as that’s considered a rough tipping point for potential fertility issues.
For more information on smoking and fertility, please download the book, Everything Egg Freezing: the Essential Step-by-Step Guide to Doing it Right or join the digital Fertility Fit™ program (beta).
Weight loss and menstrual cycle: clinical and endocrinological evaluation. https://www.ncbi.nlm.nih.gov/pubmed/1580168 The relation of obesity to menstrual disturbances. https://www.nejm.org/doi/full/10.1056/NEJM195207102470204 Impact of obesity on infertility in women. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4456969/ Obesity is associated with early pregnancy loss after IVF or ICSI. https://www.ncbi.nlm.nih.gov/pubmed/10646815 Maternal obesity and risk for birth defects. https://www.ncbi.nlm.nih.gov/pubmed/12728129 Obesity and reproductive functions. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3520133/
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