Is your BMI a gate-keeper for fertility treatment? It’s not uncommon for a fertility clinic to delay or refuse specific fertility treatments because of high-risk metabolic health. Unfortunately, one of the primary measurements used for this relates to body weight, which for many people can feel like fat shaming or discrimination against larger body sizes. But what exactly is the scientific rationale here? (Is there one?)
This topic can be stressful and triggering for many people, which is why understanding the metabolic side of your reproductive health is critical and can lead to optimizing your health in ways not always explained by your doctor.
What is BMI and why is it used in fertility clinics?
The Body-Mass Index is a measurement based solely on a person’s height and weight. Many physicians calculate BMI to help flag individuals who may be underweight, overweight, or obese. However, this flag has its limitations: It can’t actually tell us how much body fat and/or lean tissue are present. This can make a big difference considering that these tissues affect major hormonal and metabolic processes, and also that the same volume of these tissues weigh different amounts.
At Conceive Health, we look beyond the scale with fertility testing that provides a clearer picture of your internal metabolic health.
How body fat percentage affects reproductive health
The body’s fat tissue, also called adipose tissue, can give you some sexy soft curves, but these adipose cells do a lot more. They are hormonally active. Yes, that’s right: your body’s fat cells contain key hormones that regulate your metabolism, while also affecting ovarian function. These fat cells also contain enzymes that can transform your hormones. So the more you have, the more you may be altering your balance of estrogen and testosterone.
Leptin regulation and hormone function
Leptin is a hormone that’s produced by adipose cells that reports the status of your energy reserves to your body and brain. The more adipose tissue there is, the more leptin will be produced. Leptin also increases or decreases in relation to food consumption. The idea being that after you’ve had a meal you shouldn’t still be hungry. Leptin (and another hormone called ghrelin) can alter your appetite based on caloric intake throughout the day.
Leptin signals our hormone control centers, which tell the pituitary gland to release luteinizing hormone (LH) and follicle stimulating hormone (FSH): two major reproductive hormones that control follicle recruitment, growth, and ovulation. To keep the ovaries responding and functioning optimally, we don’t want to have too much or too little leptin.
What happens when Leptin is too low?
When leptin levels are low, this can slow or even prevent the release of FSH and LH. This is why menstrual cycles can stop in people who have low energy reserves (low body fat), like in cases of malnourishment, disordered eating, or overexercising without enough caloric intake. It’s like a failsafe–your body will prevent reproduction if it knows it doesn’t have enough energy to support the demands of pregnancy.
What happens when Leptin is too high (Leptin Resistance)?
High leptin commonly occurs in people who are overweight or obese when body fat percentage is much higher than required for maintaining regular menstrual cycles. If you need leptin to release FSH and LH, then why is this a bad thing? Well, when there is a constant overabundance of leptin, the body can become resistant to it. The receptors that leptin attach to can become less sensitive and the body stops transporting it and responding to it the same way.
It’s like when you’re constantly fatigued and your typical one cup of coffee won’t do, so you start drinking two cups regularly, and over time that’s not enough so you find yourself drinking more and more to get the same stimulation that you used to get from just one cup. Except instead of caffeine for stimulation, leptin resistance sends your body the incorrect signal that you’re still hungry and should eat more, which causes more weight gain (adipose tissue), more leptin, and leptin resistance.
Adding a cherry to the top of this sundae, there is also inflammation associated with having high leptin and excessive amounts of adipose tissue on the body—another factor that can affect healthy reproductive function.
Insulin regulation and hormone function
Insulin, like leptin, is a hormone that is affected by the amount of adipose tissue in the body, but it has a much different role. Unlike leptin whose job it is to monitor and send regular reports on energy reserves, insulin responds to blood sugar levels. Insulin is the key that, when inserted into its receptor lock, opens the gate to allow a shipment of glucose into the cell. When we eat sugars and carbohydrates, the pancreas releases insulin so the body can deliver these sources of fuel to our cells.
Eating sugars and simple carbohydrates too regularly and in too high of quantities can cause chronic overproduction of insulin. This can cause our cells to develop a resistance to insulin–one of the earliest stages of (and risk factors for) type 2 diabetes.
How insulin resistance impacts PCOS and ovulation
We have insulin receptors in the ovaries. Hyperinsulinemia (high insulin levels) is one of the driving factors of Polycystic Ovary Syndrome (PCOS). Excessive insulin tells the ovaries to produce more androgens, like testosterone. Too much of this signal interferes with the normal functioning of the ovaries. It can slow or prevent follicle development, egg maturation, and the timing of ovulation. This is why those with PCOS often have long and irregular cycles.
On top of this, high insulin concentrations tell your body to keep putting on more adipose tissue around your abdomen, keeping you in the cycle of having excess insulin, leptin, and adipose tissue. When you get to a certain point of metabolic dysregulation, these factors keep working against you to keep you in this state. That doesn’t mean that it’s impossible to shift, just that you may need additional support to re-train the body.
Can I still get pregnant with a high BMI?
Having either a BMI less than 19kg/m2 or greater than 25kg/m2 has been associated with a delayed time to conception, lower pregnancy rates, and higher rates of pregnancy loss. However, it’s important to separate body-weight from body fat percentage, since it’s the adipose tissue that is a major influence on the amount and effect of insulin and leptin.
It’s also important to remember that obesity is associated with chronic inflammation and oxidative stress in the body. This effect has also been reported in the ovaries. An increase in oxidative stress in the follicular fluid of obese females has been linked to follicular damage and interference with oocyte (egg) maturation and embryo development.
Pregnancy outcomes (the “Fetal Micro-environment”)
Preconception obesity and obesity in pregnancy increases the risk of developing gestational diabetes and high blood pressure. Since oxidative stress affects cell mitochondrial function and embryo development, this impacts pregnancy health, viability, and overall fetal development.
The micro-environment around our eggs and the embryo impacts the growth and health of these cells. Meaning that inflammation and metabolic dysfunction can affect the “epigenetics” of the fetus. The environment around the embryo and fetus influences gene expression: which genes are activated, to what degree, and when.
IVF success rates and BMI cut-offs
The concerns about going through advanced reproductive technologies like IVF Support with a high BMI are valid. In one study, having a BMI greater than 30 (compared to those with a BMI <30) decreased the odds of a live birth by up to 68%.
In addition to this, children of mothers who were obese or had insulin resistance or type 2 diabetes preconception have a significantly higher risk of developing these conditions, as well as potentially impacting cardiac function, immune system programming, and neurological function. Although it may seem limiting to have a BMI cut-off, healthcare practitioners want you to have the best possible chance at success–not just a positive pregnancy test, but a viable and healthy pregnancy, and a healthy child.
Next steps: Improving metabolic health for fertility
At Conceive Health, we are here to help you achieve your goals. Spending three to six months on Preconception Healthcare to better balance your hormones is a great investment before undergoing major fertility treatments.
The best way to get a handle on insulin, leptin, and adipose management is with diet and lifestyle factors, including regular exercise. But we understand it’s not always this easy. If you have genetic factors or other disorders that affect your ability to lose weight, we can help! There are also many supplements that increase insulin sensitivity to help improve your ovarian function.
Get started with a complimentary 15-minute discovery call with a fertility naturopath, book online today.
You can also browse our Frequently Asked Questions or learn more about our Metabolic Fertility Program.
References
- Gu L, Liu H, Boots C, et al. (2015). Metabolic control of oocyte development: linking maternal nutrition and reproductive outcomes. Cell Mol Life Sci. 72(2): 251-71
- Silvestris E, Lovero D, Palmirotta R. (2019). Nutrition and Female Fertility: An Interdependent Correlation. Front Endocrinol (Lausanne). 10:346
- Moragianni VA, Jones SM, Ryley DA. (2012). The effect of body mass index on the outcomes of first assisted reproductive technology cycles. Fertil Steril. 98(1):102-8
- Pavlović N, Križanac M, Kumrić M, et al. (2025). Obesity in reproduction: Mechanisms from fertilization to post‑uterine development (Review). Int J Mol Med. 56(6):204
- Rich-Edwards JW, et al. (1994). Adolescent body mass index and infertility caused by ovulatory disorder. Am J Obstet Gynecol. 171(1):171-7
- Moschos S, Chan JL, Mantzoros CS. (2002). Leptin and reproduction: a review. Fertil Steril. 77(3): 433-44
- Grodstein F, Goldman MB, Cramer DW. (1994). Body mass index and ovulatory infertility. Epidemiology. 5(2):247-50
