“SEVERELY obese women should not have IVF on the NHS.” This headline was prompted by the British Fertility Society, which said that obese women are less likely to become pregnant and, if they do, are more likely to experience health problems.

It has triggered much debate about entitlement but little light has been shed on why overweight people may be infertile, and why it’s wrong to say that it’s simply their own fault if they can’t have babies.

The relationship between fertility and body weight is complicated. At its core is fat itself, which produces a whole range of hormones. An especially important one in fertility is called leptin. The more fat you have, the more leptin you produce. And, as long as your brain gets messages that there’s leptin around, it assumes that you are not starving and are fit for pregnancy.

On the other hand, if you have too little fat — say because you’re an athlete or are anorexic — the traffic of leptin messages is greatly reduced. The brain reacts by shutting down any chance of reproduction and menstrual periods disappear. So having a certain amount of fat is important for fertility.

However, if you have too much fat, and lots of leptin, it interferes with fertility on a grand scale, disrupting the function of the ovary, the lining of the womb and even embryo development. This is perhaps why the obese are so much more likely to miscarry.

And there’s yet another problem with fat. Fat cells can turn hormones from the adrenal gland into a weak form of oestrogen. For the ovary, which normally produces oestrogen in waves to a monthly timetable, this extra oestrogen is deeply confusing. The peaks and troughs of normal cycles are flattened, causing irregular cycles, or even no cycles.

And infertility.

Some poor women get a double whammy. They have hormone conditions that make them both fat and infertile. Thyroid disorders, for instance, can cause both irregular cycles and obesity. An inherited problem called polycystic ovary syndromeis usually diagnosed because women who have it are overweight, have excessive hair and fertility problems.

These women will struggle to lose weight. If they could, they would. They need sympathy and help, rather than someone making them feel even more stigmatised. What’s needed is for women to realise that obesity may hamper their chances of pregnancy and to seek help as early as possible.

That way their problems can be identified and supported properly, before it’s too late.

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