Several studies have reported that elevated body mass index is associated with a need for higher drug doses to stimulate ovulation, changes in oocyte morphology, reduction in fertilization and implantation rates, reduced rates of clinical pregnancy and live births, and increased miscarriage rates. These findings have led to growing concerns regarding obese or overweight women undergoing infertility treatment. However, the other extreme of body weight, underweight (body mass index/BMI <18.5), also warrants attention.