Introduction
The number of retrieved oocytes is critical to the success of IVF treatment. As a result, the aim of ovarian stimulation for IVF is to maximize the number of oocytes available for fertilization (1, 2). In order to achieve this goal, ovarian stimulation prior to IVF requires the administration of exogenous gonadotropins to support multi-follicular growth until the day of final oocyte maturation (3). Currently, ultrasonographic determination of the antral follicle count (AFC) and/or the measurement of Anti-Müllerian-Hormone (AMH) prior to stimulation start assist in the determination of the optimal gonadotropin dose to prescribe, the identification of patients at risk of developing ovarian hyperstimulation syndrome (OHSS) (4) or a low/no response during stimulation (5). Despite the assessment of these parameters as an indicator for the expected treatment response prior to cycle initiation, close monitoring of individual response to ovarian stimulation is mandatory to avoid treatment complications, facilitate individualization of treatment and assist in the determination of the optimal day for final oocyte maturation and oocyte retrieval.
Routine monitoring of ovarian stimulation for IVF/ICSI includes transvaginal ultrasound examinations (TVUS) and measurement of serum estradiol (E2) (6). With TVUS, ovarian response to gonadotropin administration is monitored by recording the size and number of developing antral follicles and serum E2 levels that reflect the collective hormonal capacity of the follicles.
In a natural cycle, aromatase activity begins to increase on cycle day 5–8 in follicles larger than 8 mm (7, 8). Upon selection, the dominant follicle in a natural cycle produces more E2 than the other follicles and the E2 level increases with the increasing size of the dominant follicle.

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