Women need to have a BMI below 35 to be referred for fertility treatment.
In women under 37 years with normal levels of folic-stimulating hormone, a BMI below 30 should be the aim.
A healthy weight is needed to ensure women have a good chance of success and to mimimise the risk of complications, states the BFS in its latest guideline.
GPs need to tell obese women wanting to be referred for fertility treatment that carrying excess weight reduces the likelihood of successful conception both naturally and with assistance.
Additionally, being obese increases the risk of complications in pregnancy, including miscarriage and congenital abnormalities.
The safety of fertility treatments is also compromised by obesity, which makes anaesthesia difficult and can mean the ovaries are unable to be seen on ultrasound scans.
Psychological support, dietary advice, exercise classes and even pharmacological and surgical procedures should be offered to these women.
Losing just 5-10 per cent of their body weight could restore fertility, according to the BFS.
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