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BMI and Fertility

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Being at a healthy weight before pregnancy can boost the odds of a smooth pregnancy and fewer complications during labour and birth — as well as a healthy baby. It can also increase your chances of conceiving.

In Canada, 25% of assigned females at birth of reproductive age are overweight, and another 19% are obese. It’s more likely that individuals with higher BMIs will experience fertility challenges. So, what is BMI? And what should you know about BMI and fertility? Below, we take a closer look at what all of this means for getting pregnant.

What Is BMI?

BMI stands for Body Mass Index. It’s calculated by dividing a person’s weight in kilograms by the square of their height in metres. If you don’t want to do the work yourself, there are also various online BMI calculators that can help you determine your individual BMI.

So, what does your BMI mean? Generally, BMI categories are used to assess one’s risk of health problems.

The BMI categories include:

Below 18.5 — Underweight
18.5 – 24.9 — Healthy weight
25.0 – 29.9 — Overweight
30.0 and above — Obese

It’s important to note that being underweight can be just as unhealthy as being overweight. Additionally, some individuals with higher muscle mass may fall into the overweight category but not be considered (or look) overweight. However, this scenario is more likely for elite athletes or bodybuilders.

Does BMI Affect Fertility?

The short answer: Yes, it can.

BMIs at the extreme ends of the BMI category ranges — such as significantly underweight, significantly overweight, or obese — tend to coincide with infertility. Generally, getting closer to the healthy BMI range is associated with improved fertility and pregnancy outcomes. So, with that in mind, let’s take a closer look at how a high BMI and low BMI impact fertility.

Does High BMI Affect Fertility?

It’s not exactly a secret that a higher body weight can impact overall health and increase your risk of chronic and life-threatening diseases. When our overall health is at stake, our reproductive system is also frequently impacted. Having a higher BMI is no exception.

Research shows that overweight and obese women are at a higher risk of menstrual dysfunction and anovulation (a lack of ovulation). However, it’s worth noting that getting pregnant when overweight isn’t impossible. It may take longer to get pregnant, and you may be at an increased risk of miscarriages and other pregnancy complications. For instance, women with a BMI of 35 or higher have a 25% increase in first-trimester loss compared to women of the same age.

Does Low BMI Affect Fertility?

Having an extremely low BMI isn’t ideal for fertility outcomes either. Research states that women with low BMIs are also more likely to have infertility problems and negative pregnancy outcomes. According to studies, a lack of fat leads is often associated with low estrogen, as well as disruptions in a normal menstrual cycle and ovulation. Inevitably, this may make it difficult to conceive naturally.

Ideal BMI For Fertility

Since low and high BMIs may impact fertility, it’s recommended to achieve a healthy BMI — or get as close as possible to a healthy BMI — before pregnancy or trying to conceive. Starting from a place of good health is more likely to lead to favourable outcomes and a healthy and happy baby.

BMI And Fertility Treatment

If you’re struggling to get pregnant, there are many options outside of natural conception. These include in-vitro fertilization (IVF) and intrauterine insemination (IUI). Yet, your BMI may impact the outcomes of these fertility treatments.


Compared to individuals with a healthy BMI, those with a BMI over 25 have a lower chance of getting pregnant through assisted reproductive technology (ART), including IVF. Research suggests that these women require higher doses of hormones during this process and have higher incidences of miscarriages.

Ovarian response during IVF is further compromised in obese women, impacting oocyte retrieval and embryo quality. Obese women also experience lower implantation rates and higher miscarriage rates, leading to decreased live birth rates.

Studies have explored weight loss as a fertility treatment for those who are overweight and obese. Researchers indicated that miscarriages dropped from 75% to 18%, suggesting that losing weight may prove a viable pathway for overweight or obese individuals experiencing infertility.

Therefore, obtaining a healthy weight beforehand could save you both time and money when it comes to IVF.

BMI Limit For IVF

According to data collected from Canadian IVF directors, the median upper BMI limit for IVF was 38. However, researchers propose an upper and lower BMI cutoff for improved fertility outcomes, with the average lower limit suggested at 17.3 and the average upper limit suggested at 40.

Currently, however, there is no set cut-off. Instead, this is left up to your fertility team to decide. For those with higher or lower BMIs, weight loss interventions may be recommended before proceeding with fertility treatments.

Ideal BMI For IVF

Ideally, it’s recommended to have a normal BMI (or close to it) before beginning IVF. This means under 25 or above 18. In turn, this can significantly improve your chances of a successful IVF cycle and pregnancy.


Similar recommendations are in place for IUI. Where IVF involves the creation of an embryo in a lab and then implantation, IUI involves the insertion of the sperm into the woman’s uterus around the time of ovulation. While slightly different, some studies indicate that a low BMI is associated with poor IUI outcomes.

At the same time, a high BMI may have similar effects. Yet, other research showed no difference between those with a healthy BMI and those with a higher BMI when it came to pregnancy outcomes and live birth rates. One study did suggest that a higher BMI is linked to a great risk for “biochemical pregnancy,” which usually means an early miscarriage (in the first five weeks) has occurred.

If you’re curious to learn more, reach out to our compassionate and knowledgeable fertility team today. At TRIO, we’re committed to helping you overcome your fertility challenges.


How does BMI affect ovulation?

A high or low BMI can lead to hormonal imbalances, which can impact the menstrual cycle and ovulation. In many cases, these imbalances may halt ovulation altogether or make it difficult to predict.

Can losing weight improve fertility?

If your BMI falls in the obese or overweight category, losing weight may improve your fertility. This can not only increase your chances of getting pregnant but also lead to healthier outcomes for parent and baby.

Does BMI affect male factor fertility as well?

Yes, being overweight can also negatively impact sperm health. Obesity can impair sperm quality, quantity, motility, and more.