Nowadays we’ve all heard of BMI: the calculation based on weight and height that gives you a BMI ‘score’. Many of us probably have a rough idea of our own BMI and know whether we come somewhere in the range of normal. We mostly use this number to calculate if we are over or under the ideal weight for our height. It is therefore associated with weight management. But there is also a great deal of evidence that BMI and infertility are directly linked in a way that may come as a surprise to some.

In this IVI blog article, we look how BMI and infertility are connected. We show you how to calculate your BMI. We also explain the various ways in which being over or under a normal weight can not only have an impact on your fertility but, if you do become pregnant, can cause problems for you and your unborn baby.

What is BMI and how do you calculate it?

Body Mass Index (BMI) is a measure calculated by dividing your weight in kilograms by your height in metres squared. So if you weigh 65 kilos and are 1.7 metres tall, the sum would be 65 / (1.7 x 1.7), with the result being a BMI of 22.5, which is a healthy weight. You don’t need to get the calculator out, as you can find plenty of online apps that will do the sum for you. Just search for BMI calculator. Here’s what the results generally mean:

• BMI of 18.5 – 24.9 is a healthy weight
• Less than 18.5 is underweight
• BMI of 25 – 29.9 is overweight
• 30 – 39.9 is in the obese range.

How can being overweight affect fertility?

The relationship between BMI and fertility is not always straightforward. Generally, having a high amount of body fat can cause disruption to the normal hormone balance. However, in some cases an underlying imbalance can cause weight gain in the first place. This is why, if an eventual link between your weight and infertility concern you, it’s sensible to see a doctor. Being overweight can cause fertility problems in several ways:

• Your menstrual cycle, including ovulation, is controlled by levels of sex hormones, mainly oestrogen and progesterone. Also by the signals they give your body to move on to the next stage of the cycle. Part of the normal function of fat cells is to store, and to some extent produce these hormones, mainly oestrogen. Anyway, if you have too many fat cells, the amount of oestrogen they store, and produce is excessive. This disrupts the normal functioning of your menstrual cycle and in severe cases can cause complete cessation of ovulation.
• Being overweight can reduce the effectiveness of assisted reproduction treatments such as in vitro fertilisation (IVF).
• Studies have shown that improvements in fertility can be achieved through reducing obesity, particularly abdominal obesity. Clinicians generally agree that weight reduction is an important primary treatment for obesity-associated infertility before trying other techniques such as assisted reproduction.

Does being underweight affect fertility?

According to the European Journal of Public Health, there are significantly more overweight people, at 53% of the population, than underweight, at only 2%. While this illustrates the difference in the scale of the problem, there are still many individuals whose fertility is compromised by being underweight, that is with a BMI of less than 18.5.

• Women who are underweight or very athletic have too few fat cells, from a reproductive point of view. In response to this, the cells produce 2-Hydroxyestrone, also known as anti-oestrogen, which can make ovulation shut down altogether. This is the reason for amenorrhea, the lack of periods, in some athletic women. They may have an apparently normal BMI and yet too high a proportion of muscle to fat cells.
• Being underweight is not necessarily an absolute obstacle to becoming pregnant. It could just mean that it will take longer than usual. The hormonal disruption may make ovulation irregular or sporadic rather than completely absent. A normal menstrual cycle can often be re-established by gaining some weight.

Are BMI and infertility linked for men?

For men who are overweight, there could be a link to changes in the balance of hormones, particularly testosterone, which are crucial for reproduction. An imbalance can result in a low sperm count and low sperm motility. A study published in the Harvard School of Public Health found that obese men had a 42% higher chance of a low sperm count than normal-weight men, and were 81% more likely to produce no sperm at all. However, it is not completely clear at this stage if the low sperm count was directly caused by obesity or by other underlying health issues.

A high BMI and pregnancy: what are the risks?

Having a BMI that is too high not only compromises your likelihood of becoming pregnant. It also brings a higher likelihood of risks to a healthy pregnancy for you and your baby. These risks include:

• Pre-eclampsia, a dangerous condition which is twice as likely for women with a BMI over 35 as it is for women with a BMI under 25;
• Thrombosis;
• Diabetes;
• Premature birth or the need for a Caesarean section.

We should emphasise that these are risks, not inevitabilities. It is perfectly possible to have a healthy pregnancy if you have a high BMI. It’s true that the progress of your pregnancy will need to be closely monitored over a course of antenatal sessions.

How to improve your pregnancy prospects

We know that there is no magic wand for people who are overweight and who may have been struggling to manage the situation for many years. However, the good news is that if you can manage to bring your BMI down by just a few points, it will have a big impact on your pregnancy prospects. The first step is to contact your healthcare professional to discuss a plan based around a healthy balanced diet and an appropriate level of exercise.

Contacting us at IVI

If you are concerned about BMI and infertility, it’s quite possible that there are other conditions underlying your weight gain and preventing you from getting pregnant. For example, polycystic ovarian syndrome (PCOS) have often to do with weight gain and could well be the cause of your infertility rather than the weight itself. At IVI we have a wealth of experience in treating PCOS with IVF and other treatments to resolve fertility problems. Just get in touch through our online contact form and together we may find a solution.