During controlled ovarian hyperstimulation (COH), human chorionic gonadotrophin (hCG) is administered to trigger the final follicular maturation before oocyte retrieval, in an attempt to mimic the physiologic effects of LH. The hCG is administered when more than three follicles ≥17mm in mean diameter have developed.

The time interval between hCG administration and oocyte retrieval is of critical importance since the time after luteinizing stimulus is a period of intense processes including the start of luteinization, expansion of cumulus cells and oocyte meiotic maturation.

The commonly practiced hCG administration time of 33 to 36 h in most IVF cycles aims to avoiding spontaneous ovulation before oocyte retrieval. However, several studies have shown that significantly more high quality embryos have been obtained with a prolonged hCG-to-oocyte interval of 38 h. It has been hypothesized that longer interval would be beneficial in improving oocyte quality and achieving optimal maturation.

Currently there are no data on the effect of the time interval between hCG administration and oocyte retrieval on IVF outcome in GnRH antagonist cycles. The aim of this study is to determine whether there is any difference in in vitro fertilization outcome when oocyte retrieval takes place at 36 h or 38 h following hCG administration

Inclusion Criteria:

• women undergoing fresh IVF cycle with recombinant FSH and GnRH antagonists
• regular spontaneous menstrual cycle (24-35 days)
• age ≤ 43 years
• FSH ≤ 20 IU/L

Exclusion Criteria:

• History of previous ovarian surgery
• Women with Stage III-IV Endometriosis
• Women undergoing natural cycle IVF