This retrospective study aims to assess the effect of adding letrozole in patients who have previously undergone an IVF cycle with either an antagonist or short protocol, with patients who have undergone a second cycle of IVF with an antagonist or short protocol without the addition of letrozole.

A limited number of studies suggest that the addition of letrozole to gonadotropins for IVF is associated with an increase in oocyte count, blastocyst count and implantation rate. These differences are presumed to be caused by an increase in intra-ovarian androgens associated with the addition of letrozole.

In patients with reduced ovarian reserve or reduced oocyte quality in the previous cycle, supplementation with of letrozole has the potential to increase oocyte count or oocyte quality, thereby resulting in more transferable blastocysts. The antagonist stimulation protocol with the addition of letrozole is commonly referred to as the SMART protocol (Stimulation with Minimal Adverse effects, Retrieval and Transfer

Inclusion Criteria for the SMART IVF protocol:

• Women between the ages of 20 and 42 inclusively
• Women proceeding with a SMART IVF protocol following an antagonist IVF protocol between 2010 and 2020 (study group) or women proceeding with an antagonist protocol following an antagonist IVF protocol between 2010 and 2020 (control group)
• Delay of 1 year between both IVF cycle

Exclusion Criteria:

• Women undergoing fertility preservation
• Oocyte donation IVF cycles
• Diagnosed severe male factor requiring surgical sperm extraction (micro-tese or TESA)

This retrospective cohort study will analyze IVF cycles in antagonist and SMART stimulation protocols during the period from 2010 to 2020.