Why does BMI matter?

weightBMI is your Body Mass Index, which is a ratio of your height to your weight. It is calculated as follows:

BMI = your weight in kg divided by the square of your height in metres. For instance, if your weight is 63.5kg and your height 1.68m, the sum would be:
63.5 ÷ (1.68 x1.68) = 22.5. So you have a BMI of 22.

If your BMI is under 20 you are considered underweight; if it is between 20 and 25 you are considered healthy; between 25 and 30 is overweight; between 30 and 40 is obese and over 40 dangerously obese.

Your BMI plays a significant role in fertility for both men and women.

For women, being over or underweight can both cause fertility problems. You need to be the correct weight in order to produce the appropriate amount of hormones to regulate ovulation and menstruation. Having a BMI of under 20 is a problem for fertility and a BMI of over 30 can affect your response to IVF treatment and reduce the success rate of a cycle.

It is not only having the right BMI that is critical for fertility. The amount of body fat you have is just as important. In healthy adult women fat comprises about 28 percent of body weight and if it drops below 22 percent ovulation could stop. Women with an average or above average body weight, or who exercise very rigorously, may have a lower body fat and a higher muscle content, which may lead to their periods becoming irregular or stopping altogether. Sensible advice for these women would be to reduce their exercise until their body fat returns to the normal range.

If your BMI is under 20 you may have problems conceiving and the risk of miscarriage is higher than normal. Your body does not have enough fat stores to sustain a pregnancy and so ovulation is shut down. When you gain enough weight your body senses that fat stores are more plentiful and pregnancy is a viable option and you become fertile again.

If your BMI is over 30 research has shown that just losing even 10% of your body weight can be all that’s needed to trigger ovulation, make your periods more regular and increase fertility. And this is particularly important if you have polycystic ovary syndrome (PCOS).

Being overweight can affect male fertility too, reducing both the quality and quantity of sperm. Having a BMI of over 25 for men is associated with poor sperm motility. Obese men also produce substantially less sperm than average which contains higher levels of abnormalities. Obviously, when both the man and woman are overweight this can have a combined effect on fertility. But the good news is that when men lose weight it results in a significant increase in not only the sperm count but also the number of normal sperm. Being overweight for men also increases the risk of having higher levels of sperm DNA fragmentation which can cause low fertilisation rates in IVF, poor embryo quality and implantation failure. We know that when BMI is over 25 fragmentation rises and becomes even more of a problem when the BMI is over 30.

If you need to reduce your BMI changing your diet to one that is nutritious and healthy and taking regular exercise is often enough to kick-start weight loss.

If you require more information about the effects of BMI on your fertility or you would like to book an initial consultation with one of our medical team, please call 01992 78 50 60 or email us on enquiries@hertsandessexfertility.com