The increasing prevalence of obesity in women of child‐bearing age is of growing concern in the health community. Obesity is associated with sub‐optimal reproductive performance; therefore, it is understandable that the number of young women with elevated body mass index (BMI) accessing assisted reproductive treatment (ART) is on the rise. Consequently, this study not only assessed the impact of BMI on fertilisation rates, embryo development and freezing during ART in women aged ≤38 years but also determined their subsequent pregnancy and delivery rates.

Data were retrospectively analysed from all cycles initiated in 2006/2007 for women aged ≤38 years. The BMI categorisations were as follows: normal – 18.5–24.9 kg/m2; overweight – 25–29.9 kg/m2; obese – 30–34.9 kg/m2; morbidly obese class I – 35–39.9 kg/m2; morbidly obese class П –≥40 kg/m2.

Obese and morbidly obese women required a significantly higher follicle stimulating hormone start dose than normal BMI women; however, they obtained significantly fewer oocytes (P < 0.05). Although BMI did not affect embryo development, morbidly obese class Π women had significantly reduced pregnancy rates compared to normal BMI women (30.5 vs 41.7%, respectively; P < 0.05). Furthermore, increasing BMI was positively correlated to increasing rates of preterm delivery (P < 0.05). Increasing BMI was also positively correlated to increasing delivery rates of singleton term macrosomic offspring (≥4000 g). Conclusion Obesity in women aged≤38 years does not affect embryo development; however, it does reduce clinical pregnancy rates in women with a BMI≥40 and increases rates of preterm labour and delivery of macrosomic offspring.