The relationship between weight and fertility is a complicated one. While there is research suggesting weight affects fertility, every human body is different. Women of all sizes have successful pregnancies and give birth to healthy children. Nevertheless, there is a consensus among many medical bodies that there is an ideal BMI for fertility, including an ideal BMI for IVF treatment. In this article, we will explain why, for women and couples who are trying to conceive, naturally or with fertility treatment, understanding how BMI affects the body can be useful.
What is BMI?
BMI stands for Body Mass Index. It is a way to measure weight relative to height. It is often used as an indicator of body fat. You can calculate your BMI by taking your weight in kilograms (kg) and dividing this number by your height in metres, squared. For example, if you are 1.65m and weigh 65kg:
65 ÷ (1.65 x 1.65) = 23.9
For men and women, the NHS provides general guidelines as to what your BMI indicates:
- Under 19 is considered underweight
- 19-24 is considered normal
- 25-29 is considered overweight
- Over 30 is considered obese
There are cases in which using BMI is not recommended. While BMI can indicate body fat measurements, it does not take into account muscle mass. Very muscular individuals, such as athletes, should not use BMI due to their muscle density.
What is the best BMI for fertility?
It’s important to remember that many women with low and high BMIs have healthy, successful pregnancies.
The ability to conceive depends on many factors, such as your age, the condition of your reproductive system and your hormone levels. While an egg cell and a sperm cell need to meet at just the right time for fertilisation, your hormones are responsible for allowing all of this to occur. If one of your reproductive hormones is out of balance, it is likely to affect the others and can cause certain hormonal functions, such as ovulation, to become disordered.
The reproductive hormone oestrogen plays a pivotal role when it comes to weight and fertility. Oestrogen is produced by the ovaries, as well as by fat cells and the adrenal gland. If you have a high BMI, your body may have a higher number of fat cells. The more fat cells you have, the more oestrogen is produced in your body. Among its reproductive functions, oestrogen is responsible for thickening the uterine lining in preparation for pregnancy. This is why, if you have a higher BMI, you are likely to experience heavier periods.
When oestrogen levels are too high, it can interrupt your menstrual cycle, leading to irregular periods and even anovulation, where you stop ovulating.
A low BMI can have just as significant an impact on fertility due to your body not having enough fuel to run correctly. When your body doesn’t have enough fuel in the form of calories and nutrients, it prioritises when and where to use energy and starts to slowly shut down the non-essential systems, such as your reproductive system. Hormone regulation begins to slack, leading to irregular or absent periods in underweight women with low BMIs.
According to the American Society for Reproductive Medicine (ASRM), a higher body-fat percentage can affect the success rate of IVF treatment and many medical bodies advise patients and practitioners that there is an ideal BMI for IVF.
What is the best BMI for IVF?
Many fertility treatment providers, including the NHS, require patients to have a body mass index within an optimal range of 19 – 30 before beginning IVF treatment. Some providers have a more limited BMI range, from 19 – 25. If you are unsure, it is best to consult your provider directly.
Although research into the complex relationship between weight and fertility is still ongoing, the detrimental impact of obesity on IVF treatment outcomes is becoming increasingly acknowledged.
A 2021 study of over 7,300 women did not find significant differences between pregnancy rates among women of varying BMI categories. However, it did uncover a distinct correlation between higher BMI with lower live birth rates and a higher rate of miscarriage. The study authors also noted a decline in the number of mature eggs that could be collected and in the number of embryos that reached the blastocyst stage.
There is also evidence that obesity can affect endometrial receptivity for patients undergoing IVF, with one study finding a link between obesity and mitochondrial dysfunction. Another 2021 study of 170 women concluded that obesity negatively affected endometrial receptivity by displacing the window of implantation.
What else can affect BMI?
Despite what some believe, being “overweight” or “underweight” isn’t solely down to the number of calories that you consume. There are several health conditions that can also affect your BMI.
Polycystic ovary syndrome (PCOS) is a hormonal condition that can cause irregular menstrual cycles and weight gain, as well as other symptoms such as excessive facial and body hair growth and fatigue.
Eating disorders, such as anorexia, can also lead to nutrient deficiencies that decrease levels of oestrogen, affecting ovulation and in a lot of cases leading to the absence of periods altogether.
What if my BMI isn’t ideal for IVF?
If you have a BMI too high for IVF, you will likely be recommended to lose 5-10% of your body weight. Losing weight, however, isn’t always the answer and we would recommend speaking to one of our consultants for a more personalised and precise diagnosis and recommendation. Depending on your medical history and unique circumstances, being underweight or overweight can have varying complications for your fertility.
There are many steps you can take to improve your health and chances of conceiving. One is getting your hormone levels tested at IVI. Knowing your Anti-Mullerian Hormone (AMH) level is just a straightforward blood test that can help your doctor determine your ability to conceive and whether your BMI is currently a contributing factor.
If you would like to book an appointment, please get in touch with us.