BMI and fertility: how important is our BMI when trying to conceive?

Talking about our weight can be very personal and sensitive. Talking about our weight or Body Mass Index (BMI) and how it impacts the chance of conceiving, even more so. Here we answer your FAQs about BMI and weight to help you prepare for your fertility journey. 


What is BMI?

BMI was created by a Belgian mathematician Lambert Adolphe Jacques Quetelet in the nineteenth century, to help indicate a healthy weight range. BMI measures an adult’s weight in relation to their height. 

You can quickly calculate your BMI here, by simply putting in your age, height and weight. To understand what your result, the World Health Organisation provides these guidelines:

  • Under 18.5 is considered underweight.
  • Between 18.5 and 24.9 is within a healthy weight range for young and middle-aged adults.
  • 25 to 29.9 is considered overweight.
  • Over 30 is categorised as obese.


Does weight affect my fertility?

There is no short answer as the relationship between fertility and weight is complex. Studies have shown that obesity can lower the chances of getting pregnant, as well as the outcomes of fertility treatment. For people who may be underweight, and have never been pregnant, researchers have found that they have an 18% longer delay to getting pregnant. However, they also found that underweight people who had already given birth didn’t experience any issues in getting pregnant again.

People with a low and a high BMI may have irregular menstrual cycles or cycles without ovulation. (If this is your experience, we would advise that you consult with your GP to look at what might be the cause of your low or high BMI in order to support you with a plan to increase or lower your weight, safely.)

As with menstrual cycles, a BMI of over 30 can affect testosterone levels. Studies have found that low sperm counts and low sperm motility occur more often in overweight and obese people compared with those within a ‘normal’ weight range (18.5-24.9).

However, it is important to recognise that people can successfully get pregnant when they may be underweight, overweight or obese. We therefore recommend that you discuss any questions with your GP or fertility doctor.


Why is there some debate around BMI?

BMI has come under scrutiny recently for not always being accurate or equitable. BMI doesn’t take into account differences of race or sex, and can’t distinguish between fat and muscle. 

Fertility Doctor Guy Gudex believes that using BMI in relation to fertility treatment is “outdated, as it doesn’t help our Māori and Pasifika communities.” A 2020 study from the University of Otago found that BMI is likely to be an inconsistent measure of obesity in Māori and Pasifika.

Across healthcare services, BMI is still considered to be a helpful indicator of identifying health risks. 

Whether we agree with it or not. To be eligible for public funding in New Zealand, Te Whatu Ora (Health New Zealand) requires that: “[A] woman’s BMI must be less than 32 and [a] man’s BMI less than 40.”

“One must understand that BMI is a crude measure of the body fat percentage and it ignores the important fact and that is fat mass,” says Repromed Medical Director Dr Devashana Gupta. “BMI does not distinguish between weight from fat or weight from muscle and bone. However BMI is the most widely used tool that is currently available, and it has been shown to correlate with fertility outcomes including costs associated with IVF drug use and increased risks in pregnancy.”

Dr Gupta believes that “until we have a better universally acceptable tool, we need to keep using BMI but in conjunction with a holistic approach. Here at Repromed, we believe in providing highly personalised care for each individual client.”


Get in touch

If you have any questions about the treatments provided at Repromed and how we can support you on your journey, we’d love to hear from you. New clients can book in for a 15 minute free phone consultation with a fertility doctor. Take the first step today.

Megan Black

Nurse Manager


Megan leads the nursing team through the continually changing face of IVF. She works in a multidisciplinary team, providing the essential organization between the doctors and laboratory and ensuring communication between all departments.

Megan started her IVF nursing career in the United Kingdom, working in two large London clinic’s before returning to New Zealand. She is also the Secretary of Fertility Nurses of Australasia.

I love working with people and see nursing as a vocation, not a job. I usually spend my downtime absorbed in a good book and planning my next travel adventure.