3 February 2020

Why your BMI matters when trying to get pregnant

Why your BMI matters when trying to get pregnant
Medical Director
Cesar Diaz-Garcia
MD PhD Assoc Prof
IVI London


The relationship between weight and fertility is a complicated one. While there is research that supports that weight affects fertility, everyone is different. Women of all sizes have successful pregnancies and give birth to healthy children. Never the less there is scientific research confirm there is a collation between BMI and fertility.

What is BMI?

BMI stands for body mass index and is as a way to measure weight relative to height and is often used as a way to predict body fat.

Your BMI is calculated by taking your weight in kg divided by height in meters squared.

For a woman there are general guidelines as to what your BMI indicates:

  • Under 19: “Underweight”
  • 19-24: “Normal”
  • 25-29: “Overweight”
  • Over 30: “Obese”


How BMI can impact your fertility?

It’s important to remember that many women with low and high BMI successfully get pregnant.

Conception depends on many factors, including the condition of your reproductive system, your age, as well as your hormone levels. An egg and sperm need to meet at just the right time for fertilization to occur. Your hormones need to be at normal levels for all of this to take place.

How oestrogen hormones you have, also plays a major role when it comes to weight and fertility. oestrogen is produced by the ovaries, as well as in fat cells and the adrenal gland. Therefore, if you have a high BMI, you may have more fat cells. The more fat cells you have, the more oestrogen is being produced in your body. Oestrogen is also responsible for the growth of the uterine lining, therefore the higher the BMI the thicker the uterine lining, and therefore heavier periods.

If oestrogen levels are too high it can interrupt your cycle, leading to irregular periods, and stop you from ovulating. According to the American Society for Reproductive Medicine (ASRM), a higher body-fat percentage can also affect the success rate of your IVF treatment.

It is important to remember that a low BMI can have just a big of an impact due to your body not having enough fuel to run properly. When this happens your body prioritises what to use energy on and starts to slowly shut down the non- essential systems, such as your reproductive system. Your hormone regulation also begins to slack which leads to irregular or absent periods.

What else can affect BMI?

Despite what some believe, being “overweight” or “underweight” isn’t solely down to the amount of calories that you consume. There are several health conditions which 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 extra 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 leads to the absence of periods altogether.

What if my BMI isn’t ‘normal’?

If you’re classed as “overweight” with a high BMI, losing just 5-10% of your body weight could have a positive impact on your fertility. Losing weight, however, isn’t always the answer and we would recommend speaking to one of our IVI consultants for a more precise diagnosis and recommendation. Being “underweight” or “overweight” can have varying complications for your fertility. However, there are steps you can take to improve your health and chances of conceiving. One being getting your hormone levels tested at IVI. Knowing your AMH level is just a straight forward blood test and this test can help your doctor determine your ability to conceive, and whether your BMI is currently a contributing factor.

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