The possible role of natural cycle and modified natural cycle in IVF

The first successful pregnancy and live birth resulting from IVF were achieved during an unstimulated natural cycle (1). Soon thereafter, natural IVF was replaced by stimulated IVF because of the very high cancellation rates of natural cycles, and stimulated treatment became the standard in IVF. However, ovarian stimulation is not free from negative consequences and risks, including ovarian hyperstimulation syndrome. (OHSS), which affects up to 5–10% of IVF cycles and can be life-threatening (2). Multiple pregnancies occur in approximately 30% of pregnancies in patients who undergo the COH protocol and this phenomenon is related to increased risks of pregnancy loss, obstetrical complications, prematurity and neonatal morbidity with long-term damage. The long-term side effects however remain largely unknown. Ovarian cancer and gestational trophoblastic disease may be associated with the chronic use of gonadotrophins (3). Nevertheless, the interest in natural IVF cycle treatment has been renewed in recent years because of the increased efficiency of IVF technology. With the increasing awareness of side effects of ovarian stimulation and better understanding of ovarian physiology in relation to ovarian follicular growth and maturation, IVF in natural cycles has gained great attention and interest for both normal responder and poor responder patients (4).