In the developed world, we constantly hear about an obesity ‘epidemic’, often linked to an abundance of fast and processed foods and unhealthily sedentary lifestyles. At the same time, there are loud alarm bells ringing about a general decline in fertility, often thought about in terms of declining sperm quality in men, but in fact applying equally to men and women. Could these two contemporary trends be linked?

In a nutshell, yes, but it is not a completely straightforward equation, and the ways in which weight and infertility are connected are not always the same. Even being very underweight, or having a very low proportion of body fat, can also bring a higher risk of infertility. Let’s take a more detailed look at the evidence for a link between obesity and infertility and the specific causes of the association. We will also consider what can be done to mitigate the negative impact, both from a general health and an improved fertility point of view.

What is the evidence for linking weight and infertility?

Given that both obesity and infertility are on the rise in the Western world, we cannot simply assume that they have a causal connection. However, there have been many medically and statistically robust research projects into the link, and there is no longer any room for doubt that the two are directly connected.

A study carried out in Amsterdam’s Academic Medical Center and published in the Human Reproduction journal found that even among women who were ovulating normally, those who were overweight were clearly less likely to conceive than women of normal weight. This reduction in fertility ranged from 10% in women with a BMI of 30 or more to 26% with a BMI of 35, to 43% for those with a BMI of 40 or more. Bear in mind that the study related only to women who were ovulating regularly, and this is not the case for many overweight women.

Why does being overweight carry an increased risk of infertility?

In some cases, being overweight can bring more direct and obvious risks to fertility such as anovulation.

• Polycystic Ovarian Syndrome (PCOS) is one of the most common causes of infertility in women and it is also associated with being overweight. It is a hormonal disorder in which the ovaries can develop multiple follicles but there is no full maturation in any of them, and fail to release eggs on a regular basis. The exact cause is unclear, but it is known that it sometimes develops in response to significant weight gain. Treatment with a weight loss regime can improve the condition and increase the possibility of conception through restoring normal ovarian function.
• Even without the development of PCOS, weight gain can trigger a hormonal imbalance which makes ovulation sporadic or absent altogether. Fat cells play an important part in hormonal balance. They store the hormones oestrogen and testosterone and if there are too many fat cells, there are too many of these hormones in the system. The balance which regulates the menstrual cycle, and therefore ovulation, works on a feedback loop; too much oestrogen leads to a surge in other hormones which can eventually close down the cycle, or at least have a detrimental impact on ovulation.
• The connection between weight and infertility is not just about too much weight. In women, it is well known that being very underweight can lead to the shutting down of the menstrual cycle, with ovulation completely ceasing as a result. This is an effect most frequently seen in professional athletes or in women suffering eating disorders like anorexia or bulimia. In the case of professional athletes, the role of fat cells in storing and manufacturing hormones that regulate the menstrual cycle comes into play. High muscle and very low fat proportions could cause fertility problems, even for a woman who has a normal weight but a very high proportion of muscle, since muscle weighs more than fat.
• The link between weight and infertility in men is less documented but some studies have found that overweight men are almost twice as likely to have a low sperm count as well as low sperm motility. One study, conducted by the Harvard School of Public Health, found an 11% increase in incidence of low sperm count in overweight men, rising to 42% in men classified as obese. They were also over 80% more likely to produce no sperm at all compared with men of normal weight. The study authors point out that, although this correspondence does not prove cause and effect, that is that weight gain leads to infertility, it is clearly associated with a lower sperm count, which itself could lead to fertility problems.

What should I do if I am overweight and have fertility problems?

Healthy living and weight management

Don’t despair if losing a significant amount of weight feels like a mountain to climb. If you were to lose 10% of your current bodyweight it could well improve your fertility, even if you are still in the overweight category. It’s definitely not a good idea to go on a drastic diet, but small changes in lifestyle, like choosing a piece of fruit over a sugar-laden pastry for a snack, can have a big impact over time. For more about healthy eating, both when trying to conceive and while pregnant, see our blog article about how to deal with being overweight.

Looking for help and advice

Since the link between weight and infertility is quite complex, you can’t assume that your issue arises solely from weight. For example, some hormonal problems bring both weight gain and decreasing fertility through preventing ovulation. It’s therefore always worthwhile to have a thorough check up of your fertility status and look into whether or not there could be other underlying causes of any problems. You can find out more about the causes of infertility on our website or just go ahead and make your first visit to IVI; it could be a weight off your mind!