This is a single center, prospective, observational study on the use of Gonal-f in controlled ovarian hyperstimulation (COH) in subjects undergoing ovulation induction/ intrauterine insemination (OI/IUI) therapy across Taiwan. It has been observed in the previous studies that COH with follicle stimulating hormone (FSH) is considered as one of the positive predictors of an ongoing pregnancy. Many studies in the past have tried to predict the FSH threshold, defined as the FSH dose on the day when a follicle is >10 mm in diameter, but no studies have been conducted to date to determine the predictive factors for a monofollicular development after COH in IUI cycles. Monofollicular growth contributes significantly to the reduction of multiple pregnancies and thus minimize the risks associated with such pregnancies. This study would provide preliminary data on the factors associated with a monofollicular development in Gonal-f treated cycles using a low dose step-up regimen.