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24/10/2023 - 4 min read

Why BMI is such a weighty problem

Care Team

Weight is such a difficult topic! Hard to define and even harder to talk about!

Many doctors report that talking to their female patients about the need for weight loss is one of their most difficult consultations. Telling a patient that her weight is contributing to her fertility problem and that weight loss is the answer is equally painful for doctor and patient. When does this stop being a fertility consultation and start being ‘fat-shaming’? Obesity can be a consequence of physiological and demographic factors as well as lifestyle choices. Overweight patients are well aware that they weigh ‘too much’ but successful weight loss is complex, demanding and SLOW!

Fertility patients often get referred for IVF with high BMIs and are desperately disappointed to be told that they will not be eligible for NHS funded treatment unless their BMI is 30 or under.  Older patients can be devastated to realise that a sensible weight-loss plan (ideally 1-2 pounds a week) to get them to that target will use up precious months that they simply don’t have. Crash diets and ‘surgical solutions’ like gastric bands may achieve rapid weight loss, but the evidence is  that the patients should be stable at their ‘new’ weight for a year before starting treatment or the response to treatment is poor.  If ‘Mother Nature’ thinks you are starving, then she is unlikely to think a baby is a good idea just now!

29% of the UK population are obese (that is have a BMI above 30) and the range of ‘healthy’ is in fact a BMI of between 19 and 25 (which only 40% of the population will have!). BMI is however a relatively poor way of looking at someone’s weight and assessing whether it poses a health risk or may be implicated in their fertility problem.

BMI is calculated by comparing the ratio of height and weight – weight in kilograms divided by height in metres squared. Apart from the mathematics involved, this number may be misleading because muscle weighs more than fat and so athletes often have raised BMIs. The National Institute for Clinical Evidence (NICE) and the British Fertility Society (BFS) recommend a BMI of between 19-30 as being suitable for fertility treatment.

Weight problems and fertility are frequently linked – in fact the majority of women with Polycystic Ovarian Syndrome (PCOS) are overweight and if they can lose just 10% of their body mass the irregular periods and absence of ovulation that is a symptom of the condition, and frequently the cause of their infertility, will improve the chance of a pregnancy occurring spontaneously.

At the other end of the BMI spectrum, young women who have recovered from anorexia or bulimia will often still have very low BMIs (15-18) and in addition to irregular or absent periods, if they do achieve a pregnancy there is an increased risk of miscarriage or a low birthweight baby.

Alternative measures of weight may be more helpful.

Some women are ‘pear’ shaped (that is their hip measurement is greater than their waist) and some are ‘apple’ shaped where they have ‘no’ waist. For the ‘apples’ this means that they have deposits of fat around their middles which have been shown to significantly reduce fertility. An ‘apple’ shape is a waist : hip ratio >0.85 and a ‘pear’ is a waist : hip ratio <0.7. An Australian study of over 500 women having donor sperm insemination treatment showed that the ‘pears’ were twice as likely to achieve a pregnancy as the ‘apples’ – 63% versus 32% after 12 cycles of treatment.

BMI studies also provide useful data about the impact on IVF outcome. A review of over 5000 IVF cycles for women of all ages showed that the chance of getting pregnant with a BMI over 30 was reduced by 25% and the risk of miscarriage was almost doubled.

Waist measurement may provide an even more straightforward measure of not just fertility prospects but of general health too. A recent UK study which assessed  the health of 300,000 people found that the ratio of waist measurement to height was a better predictor of high blood pressure, diabetes, heart attacks and strokes than BMI. Ideally we should all aim to keep our waist measurement less than half our height. So a 5ft 4in tall (64 inch) woman should keep her waist under 32 inches.

Weight also has an impact on male fertility. Men with a high BMI produce fewer normal, motile sperm in the ejaculate as a result of the ‘heat’ effect of that tummy, so women who start a diet and exercise routine to boost their ‘fertility fitness’ should encourage their partners to join in!