The effect of altering the timing of hCG administration on the ongoing pregnancy rate in patients stimulated with recombinant-FSH (rec-FSH)/gonadotrophin releasing hormone (GnRH) antagonists for in vitro fertilisation (IVF).

Progesterone elevation has been associated with prolongation of the follicular phase in GnRH antagonists cycles by two days after the commonly used criterion of the presence of at least three follicles of >or= 17 mm has been met. Such an intervention is associated with significantly lower ongoing pregnancy rates in GnRH antagonist cycles, without an apparent deterioration of embryo quality.

The adverse effect of P elevation on the day of hCG administration might be explained by the induction of differences at the histological level as well as at the gene expression level between endometrial samples exposed to varying concentrations of progesterone (P). Prolongation of follicular phase by delaying hCG administration for two days is associated with a higher incidence of endometrial advancement on the day of oocyte retrieval in GnRH antagonist cycles (Kolibianakis et al., 2005). Moreover, Vaerenbergh et al., (2011) demonstrated a distinct difference in endometrial gene expression profile between patients with progesterone serum concentration above and below the threshold of 1.5 ng/ml on the day of HCG administration.

Due to the fact that earlier triggering of final oocyte maturation is expected to result in lower progesterone levels on the day of hCG administration it might be assumed that such an intervention might result in an improved probability of pregnancy by leading to a less deranged and more receptive endometrium.

The purpose of this randomized controlled trial is to evaluate whether triggering of final oocyte maturation as soon as ≥ 3 follicles ≥ 16mm are present on ultrasound or one day later affects the probability of pregnancy in patients stimulated with rec-FSH/GnRH- antagonists for IVF.

Inclusion Criteria:- Age ≤ 39 years

• Body mass index between 18 and 29 kg/m²¬
• Presence of both ovaries
• Basal levels of estradiol (≤ 80 pg/ml) and progesterone (≤ 1.6ng/ml) on day one of the cycle
• Treatment with IVF/ICSI
• Embryo transfer on day 3 (1 or 2 embryos)
• Patients can enter in the study only once

Exclusion Criteria:

• Presence of endometriosis stage ≥3(AFS)
• Polycystic ovarian syndrome (Rotterdam criteria)
• Need for preimplantation genetic diagnosis (PGD)
• Azoospermia testicular sperm extraction (TESE)