Obesity has a significant adverse impact on reproductive outcome. It influences not only the chance of conception but also the response to fertility treatment, and increases the risk of miscarriage, congenital anomalies and pregnancy complications in addition to potential adverse effects on long term health of both mother and infant. Women should aim for a normal BMI before starting any form of fertility treatment. Treatment should be deferred until the BMI is less than 35 kg/m2, although in those with more time (e.g., less than 37 years; normal serum FSH concentration) a weight reduction to a BMI of less than 30 kg/m2 is preferable. Clinicians should consider deferring treatment to women outside these guidelines. Women should be provided with assistance to lose weight, including psychological support, dietary advice, exercise classes and where appropriate, weight reducing agents or bariatric surgery. Even a moderate weight loss of 5-10% of body weight can be sufficient to restore fertility and improve metabolic markers.