What is BMI and how does it affect fertility?

Your body mass index (BMI) is calculated by comparing the ratio of your height to your weight. During in vitro fertilization (IVF), your clinician may use your BMI to identify potential issues with your fertility and predict your chance of success with IVF.

However, while BMI is often used as a measure of health, it provides a very limited view. For example, BMI doesn’t give you a sense of your body composition (ratio of fat to muscle to bone). It doesn’t measure fitness. And it doesn’t compare your waist-to-hip ratio. All of these alternatives can offer very valuable information about your health! Importantly, BMI is just one of many factors – such as race/ethnicity, age, sex, etc. – that can be used to assess your health.

All that said, with the caveat that BMI offers very limited information, we’ll discuss BMI here in the context of its effect on IVF since it's used as a marker of health and success potential in many studies and fertility clinics.

Here are some ways that your BMI may indicate potential issues with your fertility and chance of IVF success:

BMI below 18.5

If you have a “low” BMI, you may experience suppressed ovulation, making it hard to conceive without assistance. IVF can help with this by controlling ovulation with medications. However, one study found that compared with patients with a normal BMI, patients with a low BMI experienced lower implantation rates (33.56% vs. 37.26%), lower rates of clinical pregnancy (48.14% vs. 53.85%), and lower rates of ongoing pregnancy (43.04% vs. 50.47%) (1).

BMI between 18.5 - 25

If your BMI falls in this range, you have a “normal” BMI and your chance of IVF success may not be influenced by your weight.

BMI between 25 - 30

If your BMI falls in this range, you have a “higher than average” BMI. According to one study, for patients experiencing infertility at this BMI, weight loss of 10% improved conception and live birth rates (2). As for your chance of success with IVF, there’s conflicting evidence about the effect of a “higher than average” BMI. However, one systemic review and meta-analysis of 33 studies found that patients with a BMI over 25 experienced a reduction in relative clinical pregnancy rates (by 10%), relatively lower live birth rates (by 9%), and relatively higher miscarriage rates (by 31%), as compared to patients with a “normal” BMI (3).

BMI above 30

If you have a “high” BMI, research shows that your fertility may be affected, IVF may be more risky to pursue, and you may experience lower success rates. Below, we’ll share some of the research behind this.

A high BMI may affect IVF success

More than in other areas of research related to IVF, there’s conflicting evidence about whether and how much BMI affects IVF outcomes. However, it may be helpful to know some of what the research says. Some ways that a high BMI may affect your IVF outcomes include:

May reduce the number of eggs retrieved

One study found that patients with a high BMI retrieved fewer eggs than patients with a normal BMI when using a conventional ovarian stimulation medication protocol (4).

May experience a canceled cycle

An IVF cycle is canceled when there aren’t enough eggs or embryos to continue, or when it’s not healthy or safe for the patient to continue. In one study, patients with a BMI over 30 were more likely to have a canceled cycle (25.3% cancellation rate) compared to patients with a normal BMI (10.9% cancellation rate) (5).

May need increased medications and anesthesia could be more involved

The first part of IVF, ovarian stimulation, involves the use of medications to help your ovaries grow more eggs to maturity than normal before they are retrieved in a simple, outpatient procedure. If you have a high BMI, you may be prescribed higher doses of ovarian stimulation medications (6) (unless you have PCOS, in which case lower doses of medications may be required ). You may also need higher doses of anesthesia medications, have a longer egg retrieval procedure, and undergo a more intensive procedure to retrieve your eggs (a transabdominal retrieval) (7).

Embryo transfer may be more challenging

Since embryo transfer requires the use of an ultrasound to place the embryo in the uterus, this procedure may be more difficult with a patient with a high BMI, since it will be harder for the clinician to see details with the ultrasound.

May experience a reduced overall live birth rate

A meta-analysis of 21 studies found that a BMI over 30 had a negative effect on live birth rates, especially if the BMI was related to polycystic ovarian syndrome (PCOS) (8).

Should I try to lose weight before starting IVF if I have a high BMI?

It depends! Losing weight in a sustained and healthy way may help to improve your fertility treatment outcomes. But there are other factors to consider:

Your age

If you are over 36 years old, your BMI may not matter so much during IVF treatment. One study found that for patients above 36, high BMI and normal BMI patients had similar outcomes (9). Additionally, the time it takes to reduce your body weight in a safe and steady manner may not be worth it when compared to the amount your eggs may age during that time. This is because a major factor in IVF success is your age at the time of egg retrieval.

Your other diagnoses

Some fertility diagnoses also affect your weight. Depending on the diagnosis, you may consider a weight loss program before starting IVF. (For example, if you have PCOS, weight loss may have a significant impact on your chance of success with IVF (10).)

Your fertility clinic

Unfortunately, many fertility clinics have a maximum BMI at which they will accept patients, citing concerns about safety and outcomes (11). This can discourage people from seeking the fertility care they deserve. If you have a high BMI, are worried that it might be a reason why a clinic might not take you as a patient, and live in a place with a range of clinics to choose from, we recommend contacting a selection of those until you find one where you feel comfortable and cared for.

If you’re concerned that your BMI may be affecting your fertility or chance of IVF success, you should consider consulting your clinician for a more comprehensive and individualized assessment of your health. You may talk to your fertility clinic for their recommendations on whether you should make dietary and/or lifestyle changes before pursuing IVF for your best chance of success.