We thank our correspondents Olusegun and Gerald for their interest in the study design of our article "Adverse pregnancy outcomes with assisted reproductive technology in non-obese women with polycystic ovary syndrome: a case control study" [1].

Our purpose was to investigate adverse pregnancy outcomes in non-obese women with polycystic ovary syndrome (PCOS) compared with obese-PCOS and control groups. We had analyzed our IVF data system and had conducted this study retrospectively. We were trying to clarify the characteristics of PCOS, so we distinguished between an obese group and non-obese group within the PCOS and control groups. Our control group had tubal factor infertility, which was acquired. A tubal factor infertility group is commonly selected as a control in studies investigating assisted reproductive technology (ART). We became accustomed to using the term "control group," and then just shortened this term to "control." We had selected the patient groups within the same period according to the indication for ART. We only excluded the patients with multiple indications for ART. We did not select the patients on the basis of an outcome of interest.

The correspondents pointed out that our study design did not fulfill the principles of a case control study. We have given the problem of the study design description our careful consideration. We agree with their evaluation that our study is a type of cohort study rather than a case-control study.